A combination of both a fortress and a landing pad, Castellum Stronghold was a prefabricated quickly-assembled base and defensive strongpoint used by the Legiones Astartes during the Great Crusade and Horus Heresy.
Consisting of interlocking modular fortifications, the Castellium Stronghold was intended for frontline use after being quickly deployed from orbit by landing craft. The base could be quickly assembled, even under enemy fire. The individual components were deployed in drop cradles clad in ablative armour, the remnants of which are jettisoned at the moment the sections landed. The Iron Warriors and Imperial Fists, both having a natural hard-on for fortifications and siegecraft, were known to take pride in their ability to quickly assemble the stronghold. Skilful Space Marine forces could have the base completed in as little as 30 minutes.
Castellum Strongholds were deployed for short-lived but high-intensity operations where the Space Marines needed to establish a fortified base in highly contested warzones. It served as a firebase, artillery redoubt, strongpoint, command centre, and mustering station. Sometimes the bases would be dropped far to the rear of enemy lines, causing confusion and forcing their foes to divert forces from their frontline to protect their rear.
The Stronghold itself consisted of a primary bunker and three bunker annexes. Each point was equipped with weapons emplacements equipped with Heavy Bolters, Heavy Flamers, Lascannons, Hyperios Missile Launchers, Whirlwind Multiple Missile Launchers, Multi-Meltas, or Quad Guns. Heavier weapons such as Icarus quad Lascannons and Battle Cannons were also deployed. For support systems, the bunker was equipped with shielded gate barriers, a comms relay, and a signal-jamming array.
In case you want to play an army comprised of buildings instead of tanks and soldiers. 3 bunkers and a bigger bunker, with T9 and a total of 56 wounds (3x12 and 20). This realm of battle tile (yes, it is 2' by 2') is fairly indestructible and can pack up to 5 heavy weapons, from anti-tank guns to AA or troop mulchers. The tacticus bunker has transport space for 30 guys, and the bunkers for 12 each.
Up to 15 and 10 models can shoot out of each of the bunkers. If your army doesn't fit inside completely, they can man the battlements for a cover save. And until the big bunker is destroyed all infantry, bike and dreadnoughts get a 4++ against shooting attacks while inside the stronghold. Oh, and one stratagem per turn costs 1 less CP if your warlord is embarked.
Each of the small bunkers can take one of: Twin Heavy Bolter, Twin Heavy Flamer, Twin Lascannon, Multi-melta, Twin Assault Cannon, or Air Defence Missiles. The big bunker can take up to two of: Twin Heavy Bolter, Twin Heavy Flamer, Twin Lascannon, Multi-melta, Twin Assault Cannon, Castellum Air Defence Missiles (d3 shot krak missiles with the AA rule and the ability to fire at units it can't see), Whirlwind Castellan Launcher (2d6 frag missiles at S6 with no need for LoS), Castellum Battle Cannon (nicked from the Leman Russ), Icarus Quad Lascannon (4 lascannon shots with the AA rule), or the pointless Communication Relay (FW meant that the -1CP to strats bonus only occurred when you took this, but they named the special rule "Command Relay" instead of "Communication Relay" so it comes as standard instead). All these guns must target the nearest enemy unit unless there's a friendly unit embarked within. At 550 points without guns this is something of an apocalypse thing.