[Templar Mikeel]

The Imperial Titan Legions are the most powerful fighting arm of the Adeptus Mechanicus, physical manifestations of the God Emperor's might. Each of the Forge Worlds founded by Martian colonists in the pre-Imperial era is guarded by a Titan Legion.
A Legion typically numbers 16 machines at full strength with a combination of the Titans shown below; however this figure varies greatly in practice. Battle losses can take centuries to replace, as the construction of a new Titan is a long and laborious process.
Titan Legions are deployed only in the most vital campaigns and on battlefields that will allow them the room to maneuver and unleash the full power of their colossal weaponry.

The first Titans were built during the Age of Strife on Mars, as a way for the then-new Adeptus Mechanicus to effectively fight on the irradiated surface. They were later sent with the fleets that established the first Forge Worlds, leading to the creation of the Titan Legions - armed forces made up entirely of Titans specific to that forge world. There are a variety of classes that a Titan falls under.
Due to their high standing, all Titans are important enough to be given their own High Gothic names when built. This is represented in published materials by giving each one a two-word Latin name, always some venerated title or concept. Examples include Aquila Ignis, Regalis Annihilatus, Vitas Falco, Imperius Eradicus, Imperius Belladon, Apocalyptus Credo, Imperius Brontos, Imperius Excelsior, Dies Irae, Imperius Augusta, and Dominatus. Similarly, the loss of a Titan is considered tragic and mourned by the Forge World that produced it.
Imperial Titans are unique in the sense that their computer mind-cores are sentient, albeit in a highly-erratic manner. This is in contrast with the usually non sentient, machine spirits, or on board computer systems found in many other Imperial war machines, such as tanks and other armored vehicles. In the comic, Titan, the newly connected Princeps was formally greeted by Imperius Dictatio inside of its mental landscape; there was also Laudator Magnificat, which was forcibly reactivated after falling in combat. This proved disastrous: the mind-core had suffered severe psychological trauma from its "death", and began attacking everything.
Most Imperial Titans are roughly humanoid, but with thick lower legs, a hunched head, and an over sized torso. Forward-oriented main weapons mount directly into where the arms would be, and additional weapons are often found on the chassis. These configurations (called patterns) vary, and most Titans are built on these presets.

Imperial Titan Classes

Warhound Titans
Warhound Titans are the scouts or flanking elements of a Titan force. Appearance-wise, the Warhound differs the most from other Imperial Titans; anthropomorphic canine-like overall structure, reverse-jointed legs; and direct shoulder weapon mounts. They are lightly-armed and armored for Titans, although still fearsome compared to more standard combat vehicles. Due to this, they are the only ones to be normally deployed in packs of at least two members, as compared to the larger Titans which usually operate alone.
The Warhound is the smallest and most agile Titan. Standing 21 meters (63 feet) tall, the design is dwarfed by all other Imperial titans.
With the deployment of a full Legion, Warhounds are commonly deployed to reconnoiter ahead of the Battle Titans and draw enemy war machines towards them. Despite its designation as a scout, the class is formidably armed. Two weapon mount hard points are available, and armament is tailored on a mission specific basis from the following list:
Plasma Blastgun
Vulcan Mega-Bolter
Inferno Gun
Turbo Laser
The Warhound's full crew numbers 12 (this includes 4 servo-skulls). In the forward cockpit the Princeps and two Moderati pilot the machine. Two Techpriests and three Servitors attend to the engine and reactor, as well as making emergency repairs in the heat of battle.
The compact dimensions of the Warhound Titan make it the only design really suitable for urban combat operations. In such confined areas the firepower of the class becomes truly terrifying, and the enemy often lacks weaponry capable of damaging the Titans.
Two Warhound Scout Titans are also used to protect the sole entrance to the Golden Throne, the God Emperor's resting place.

Reaver Titans
The Reaver Titan is a medium-sized Titan and the smaller of the two Battle-class Titans. They are considerably more durable than Warhounds, and are able to take roughly twice the amount of damage of a Warhound (according to the game rules). This extra durability is probably due to the unusual placement of the plasma generator on the back of the main carapace, where in most Titans the generator is located in the core of the Titan, hence the term "energy core." This placement ensures that the titan can take more frontal damage than other titans of the same approximate class, but is at a severe disadvantage when in cluttered environments (cities or canyons). It pre-dates even the mighty Warlord, the date of its original manufacture only known in the deepest sealed archives on Mars and to a handful of Techno Magos in the Titan Legions. More lightly armed and slightly smaller than the Warlord, the Reaver is better suited to supporting armoured assaults and breaching enemy strongholds. With a carapace mounted multi-launcher, the Mars pattern Reaver sends salvos of heavy missiles streaming at the distant enemy before pinpointing their armor with powerful Turbo Lasers. Having only slightly fewer void shields than the Warlord, it is usual for Titan Battle groups to field a number of both classes. Due to its age and heritage, the Reaver is slowly becoming a testimony to the power and grandeur that the Imperium of Mankind once held. Because of this each Reaver is an ancient artifact valued beyond measure, the loss of which is a sore blow to the followers of the Machine God. The Reaver Class Titan stands 28 meters (85 feet) high, and is usually manned by a crew of 16. The weapons mounted are variable; they can mount some of the more powerful Battle Titan weapons, although Scout Titan weapons are more common. Most of the patterns carry Multiple Rocket Launchers on their carapace mounting. Unlike Warhounds they can take close-combat weapons, which are extremely effective in Titan-scale mêlée battles. However, it is suggested that Reavers are becoming increasingly rare, and it is possible that the secrets to their manufacture have been lost (a common fate for Imperial technologies). For these reasons, Reavers usually perform fire support, avoiding direct combat.
The Mars Pattern's carapace mounts a Multiple Rocket Launcher and Turbo-Laser Destructors on each arm. The Lucius Pattern's carapace bears a Rocket Launcher, with the arms carrying a Volcano Cannon and Gatling Blaster. There is also a fire support variant of the Lucius, with Multiple Rocket Launchers on all three mounts

Warlord Titans
The Warlord Battle Titan is probably the most numerous of the Titans deployed by the Legions, and is second in power only to the rare Imperator and Warmonger Titans. Its primary armament consists most commonly of a Volcano Cannon on the port side weapons arm, a Mega Cannon on the starboard arm, and two pairs of Turbo Lasers mounted above and each side of the bridge. Secondary armament consists of a variety of point defense weapons, ranging from smaller Missile Launchers and Multi-lasers all the way up to large energy weapons in ball mounts. Armament on some Titans is known to be modular, such that huge Chainsaws or strategic missile launchers can be mounted in place of the common weaponry.
Warlord Titans are the larger and more powerful class of Battle Titans, the other being the smaller Reaver. They appear to be the most common class in use, and many Forge Worlds field their own patterns.
They are just under twice the size of the aging Reaver class, putting its height at around 48 meters(144 feet) . The Warlord Titan is also able to withstand considerably more damage. They have four Battle Titan weaponry hard points on the shoulders and arms, allowing a great variety of configurations. While Scout-class weapon batteries can be mounted, Warlords usually mount Battle-class weaponry. There is even a specialist class, the rare Psi-Titans, where psychic weaponry is added.
The Mars Pattern mounts two Turbo-Laser Destructors on its carapace, with a Volcano Cannon and Gatling Blaster on the arms. A special variant carries a close combat Powerfist and Gatling Blaster on its arms, plus Vortex missile and Rocket Launchers on the carapace.

Emperor Titans
Mightiest of all Titan classes, the Emperor class Titans stand 281 meters tall. Emperor class Titans are divided into two classes, the Imperator and Warmonger. Of these two, the Imperator is a more general-purpose assault platform, whereas the Warmonger is a dedicated fire-support unit with advanced fire control and targeting systems. Both are extremely powerful. They are loaded to the brim with pre-installed weapons, and their mounts can handle Titan weapons too big for even the other classes (they do not carry any of the close-combat weapons, as their sole focus is on extreme firepower). Emperor Titans are very rare, and can crush many weaker war engines. This class of titan also houses companies of troops on board for defense and assault assistance.
The most noteworthy feature of an Emperor titan is the fortified castle structure that occupies its entire upper half. These castles sport ornate features like spired towers and stained glass windows. The building is heavily-armed, and more than capable of contributing to combat. Due to its prominent position, the Titan's head is hunched more than normal on the body, almost level with the arm weaponry.

Knights
Knights are smaller than Titans though built on the same principle, and are also related to Sentinels. They can be seen as one-man mini-Titans. Unlike Imperial Titans, Knights use high-capacity energy cells rather than plasma reactors. Knights also benefit from power fields, which provide the same protection as Titans' void shields though they cannot be replenished in the course of a battle.
They are the product of feudal agri-worlds within the Imperium and are, like the Sentinels, in imitation of machines built by the Eldar - "Eldar Knights". While Sentinels herd sauropoda, Knights defend them from predators.
There are several types of Knights- the Warden, Paladin, Lancer, Errant, Baron and Leviathan.
The Wardens are defensive; armed with long range heavy weapons and well protected. The Paladin are fitted with a ranged weapon and a close combat weapon.
The Lancer has only a single major ranged weapon but is speedier than the other types.

Imperial Titan Weaponry and Defenses
The weaponry carried by Titans is usually dedicated to certain tasks. Some are for targeting high-priority and well-armored enemies, such as fortresses or armored divisions; others are particularly effective against legions of troops and laying down devastating artillery barrages. Titan weapons are divided into two rough categories:
Scout Titan Weapons
Scout Titan weapons are weapons normally fitted to light Titans such as the Warhound. Rarely found on the largest Titans, and usually of short range, these are designed to destroy light to medium vehicles quickly, and to deal with huge masses of troops. Below are some examples of such weapons:
Inferno Cannon - A larger version of the flamers mounted on vehicles, the triple-barrel Inferno Cannon is immensely devastating in close quarters against infantry and light vehicles, particularly those in hiding.
Plasma Blastgun - The lightest plasma-based Titan weapon and the most powerful that can be mounted on the Warhound. It is slow to fire and only medium ranged, but can badly damage well armored vehicles, and even other Titans.
Turbo Laser-Destructor - A Titan-scale laser weapon for anti-tank support. It is very destructive compared to most vehicle and infantry-level laser weapons. It's less powerful than the Plasma Blastgun, but much longer-ranged and can shoot at two different targets. Unlike the other Scout Titan weapons, Turbo-Laser Destructors are commonly found on the Battle Titans.
Vulcan Mega-Bolter - A Titan-scale bolter weapon, comprising of two large-caliber gatling gun-style assemblies. Able to tear through infantry and light vehicles with ease.

Battle Titan Weapons
Battle Titan weapons have more destructive and longer-ranged weapons commonly mounted on Reaver and Warlord Titans. Most are designed to destroy other Titans, engage enemy detachments or bring devastation to fortifications
Close combat weapon - Although they do not provide ranged firepower, these types of weapons are horrendously effective against enemy war engines and Titans - that is, if the Titan can get close enough. They can only be mounted on the Titan's arms, meaning that Warhound Titans cannot use them. Close combat weapons can include the powerfist, laser burner and chainfist.
Gatling Blaster - A larger projectile weapon commonly found on battle Titans. It is a rapid-fire weapon, like the Vulcan Mega-Bolter, but also powerful enough to inflict damage upon heavily armored targets.
Hellstorm Cannon - Seen lying on its side in Dawn of War: Dark Crusade, the Hellstorm Cannon is a huge five-barreled energy weapon, easily capable of destroying anything from light to heavy vehicles.
Melta Cannon -A larger version of the standard infantry melta, mounted on Warlord Titans. Notably short-ranged, but incredibly effective at penetrating armor and destroying Titans.
Multiple Rocket Launcher - Long range artillery which unleashes heavy barrages to suppress and devastate enemies. And because it is mounted on a Titan, it is much less vulnerable to air and fast attacks than dedicated artillery companies.
Quake Cannon - A very, very large artillery piece that is of a more massive caliber than any other artillery weapon in the game.
Volcano Cannon - Powered directly from the Titan's plasma reactor, the Volcano Cannon has extreme range and is very effective at penetrating armor, making it used primarily against war engines and other Titans. A favored weapon of the Warlord and certain Reaver patterns.
Plasma Annihilator - This is the heaviest plasma weapon mounted on a Titan. The Plasma Annihilator is devastating against any armored units or vehicles.
Missile Weapons
Warlord titans can carry one missile on a shoulder hard point, Warmonger and Imperator classes can carry them, but this is unusual.
Vortex Missile - This missile creates a warp vortex, which can destroy tanks and infantry with great ease. Vortex missiles can either collapse on themselves or move, meaning massive damage can be caused even if the missile does not hit the enemy directly
Titan Warp Missile - Titan Warp missiles ignore void shields, which make them very effective in Titan/Titan engagements.

A number of other weapons and add-ons for Titans have been described. Among them are the "Corvus" assault pod and "Deathstrike Cannon".

Titan Defenses
Imperial Titans become increasingly well-armored as their size increases. They also have 'Void Shield' generators to augment existing protection, which are able to absorb destructive energy on the Titan's behalf, although there is a limit to how much they can handle. They are directly controlled by the Princeps, and can be left running after having absorbed dangerous amounts of energy to allow for extra protection; this however has a high chance of permanently damaging the generator.
Void Shields always come in sets to maximize the likelihood of at least one remaining up after a barrage, and larger Titans may have as many as six or more fields. It is theoretically possible for a huge amount of small-arms fire to bring down a shield, giving infantry some ability to attack them. They however cannot defend against everything; for instance they cannot block non-ranged weapons (e.g. Titan-scale mêlée combat) and special technology like the "Warp missile", which bypasses normal defenses by traveling through the Immaterium.
Emperor-class Titans are also commonly equipped with point-defense weapons to protect themselves against infantry and aircraft that cannot be targeted with larger-scale weaponry. Carapace-mounted multilasers are often employed in this respect, as are more exotic weapons for dealing with aircraft.

Imperial Titan Construction
Very little is known about the methods used in the construction of Imperial Titans. It is known that many Forge Worlds are capable of producing their own Titans (and some have their own patterns and legions). Of these Forge Worlds, it is known that Mars and Lucius have their own, and it is likely that other large Forge Worlds such as Ryza and the recently destroyed Gryphonne IV do or did as well.
Often, the Imperium does not have the means to build new Titans, as the knowledge of their construction is lost. Titans however were not built using STC templates, and as such the Adeptus Mechanicus knows fully how to build at least some of the Titan variants. As the process of building any machine requires a complex ceremony and rituals, it is probable that the construction of a Titan is an immense undertaking. Such an undertaking would take decades or centuries to complete, requiring extremely experienced and talented fabricators and the purest and most blessed materials.

Imperial Titan Crew
The 'Captain' of a Titan is called the Princeps, a rare individual in that he can retain his sanity after connecting with the 'Machine Spirit' of the Titan. The Princeps controls the Titan by means of a mental link device. Using this, the Princeps merges their consciousness with the Titan's sentient machine spirit. After doing so, they can guide its movements and weapons as though it were an extension of their body, and provide a greatly heightened perception of the battlefield. It requires a strong will to maintain this connection, and Titans can go out of control on rampages of destruction should the Princeps fail to maintain dominance.
There have also been noted rare instances where a Princeps has died while connected to the Titan, thus creating a ghost of them that remains inside the Titan's 'machine spirit'. This is often referred to as a "ghost in the link".
The Princeps is located in the head, along with several system controllers called Moderati. The number of moderati depends on the size of the Titan, and their duties are relegated to monitoring and maintaining weapons, as well as keeping track of Auspex readings.
Further crew includes engineers (Techpriests) for repair work and blessings, and servitors set on the more mundane tasks like manipulation of controls and monitoring of Auspexes.

Standard Template Construct Titan
In the novel Dark Adeptus, the autonomous Castigator STC Titan was revealed, claimed to be the first one ever built. Called the Father of Titans, it was first designed during the Dark Age of Technology but became forgotten as time went by, bearing advanced long-lost technologies such as rapid self-repair routines, and a muscle-like locomotive system. Standing larger than even the Imperator-class Titans, the Castigator notably had its head on top of its body, matched by an elegant design and impossibly-graceful movement, and it was far superior to any class of titan utilised by the Adeptus Mechanicus. In the novel, it was armed with a powerfist and multi-barreled cannon that fired daemons.
The single Castigator-class built in that novel was brought down by destroying its reactor, and its STC, although complete, had to be destroyed due to its corruption.

Sources:
Wikipedia
Lexicanum