Fortress-Monastery

From memory (if anyone can expand on this with sources, please do so)

Fortress monasteries are the Space Marine equivalent to a hive city. They generally considered to be somewhere between New York City and the British Isles in size (correct me if I'm wrong). Black Templars and Imperial Fists both are space marine chapters who, instead of having a home world, base their operations from space-bound fortress monasteries. The Imperial Fist fortress monastery is actually gauntlet-shaped, and resembles a giant armored fist (with spires rising from the "back" of its hand, and sensorium nodes extending from most other surfaces). The Black Templars supposedly use a more conventional looking fortress monastery that simply appears as an oversized Gothic-style cathedral. Several chapters have land-locked fortress monasteries, such as the Ultramarines, whose fortress monastery is located amongst the mountains of McCragg. The Dark Angels keep a fortress monastery in the ruins of their homeworld.

Chaos has many fortress monasteries, each dedicated to at least one if not all the Chaos Gods. The key to being a fortress monastery is, each fortress monastery has the means to facilitate and habitate all 1000 (or more) space marines in a chapter. The Chaos legions have multiple fortified shrines, some mildly twisted by daemons and warp magic, others simply saturated in the unholy powers of the warp. One fortress monastery above all others stands apart from the crowd, though. The Imperial Palace of Holy Terra. Stretching across the Eurasian continent, the Imperial Palace is easily the largest fortress monastery in the Imperium and possibly in the galaxy (normally, obviously Chaos might sometimes make theirs larger, or smaller... possibly even pinprick size just to mess with the inhabitants).

The basic criteria required for a Fortress Monastery are the following:

1) Be man-made. Tau and Eldar don't make Fortress Monasteries... Tyranids gnaw on them and Necrons try to vapourize them one gauss blast at a time. Orks have Four-triss Mon-a-Stair-eez, but these tend to be ramshackle scavenged parts of tanks and buildings, or more often just elaborately modified Roks.

2) Be larger than the largest Imperial starships. It doesn't matter that some of the starships out there are 12 kilometers long, 4 kilometers tall and 5 kilometers thick. If you're not bigger than that by at least a factor of 2 in all three dimensions, you're not a fortress monastery.

3) Don't just be layered or armored with guns and turrets. Bristle with them... Let not a span of 4 meters go past without at least a lascannon or heavy bolter sponson, and for every ten of these, one earthshaker, demolisher or whirlwind-launcher pod.

4) In most cases, be gothic. Very few chapters go for undecorated, flat and blank exteriors and interiors. That's the job of the black ships of the Inquisition... Have pointed arches, culverts with statues that double as turret defense stations, flying buttresses, even if you're a space-borne zero-gravity-environment ship the size of Iceland. The more elaborate, gold-plated, stoneworked your exterior, the more impressed we'll be when we see you, even if you belong to the Rainbow Marines chapter (their armor is pink, their shoulder pads have a rainbow, and I didn't make them up).

Finally 5) Belong to a space marine chapter or a chaos marine legion. Don't care if you were made by menials, imperial artisans and techpriests, if you're not in possession of an Imperial Space Marine chapter or a Chaos Space Marine legion, you're just an oversized cathedral. With guns.

Exemption to the rule: Sisters of Battle. Fortress monasteries are something that no Imperial Guard force could ever hope to own, but since the Adepta Sororitas have friends in such high places as the Inquisition, they can afford to both possess fortress monasteries and call them such without a Space Marine balking at the thought. (Also, the Astartes know better than to get on a Sororitas' bad side.)