The Citadel Class Ship a wonderful hunk of metal that has been used to dominate entire worlds and sectors both by the fire power it can unleash, but more importantly the amount of goods it's storage halls can hold, The amount of ships it can service, and it's ability to be a mobile city, port, and fortress.
It is one of the few 'Super Ships' not made necessarily for military use seeing as the first Citadel class was designed by a Mercator lord (merchant) to dominate the markets of more hostile sectors with a relatively safe base of operations.
Citadel Class Ships are classified in the Empire as, and I quote, "Mobile Hive Cities" and they earn that title rightfully so. Within their halls are enough room to build cities and grand statues, cathedrals, and massive merchant quarters. The Citadel Class Ships even spot many fine spires on the upper sections of the ships and a equal amount of villas and palaces for the nobles.
These ships are, technically, mobile they lurk at the speed of a sloth and only obtain 'decent' speeds once inside the warp that means once these ships reach their destination they tend to stay and very rarely move. Most become space stations with many expansions and smaller vessels and asteroids added to the ships hull or connected by bridges and other structures.
Citadel Class Ships are just as hierarchical as the Empire outside. The ship is split into many sections dedicated to certain social class within the poor living in the dark under belly hardly seeing the light of day and the rich living in spires stretching miles above basking in the glory of space. The Ship also has elements of every other social class and Adepta within it's ranks (seeing as it is considered a city) it has it's own noble families, fiery preachers, and every cruel Judges all mixed in with the merchants of a thousand worlds and criminals from all walks of life.
But numbering more then the citizens and residents are the mass multitude of slaves who serve a variety of roles from keeping the ship running (most of the official crew is slaves), private slaves of nobles, merchants, or many others, or cattle waiting to be sold within the markets. Alongside these are many beasts as well and uncountable fighters all destined for the pits to fight for the enjoyment of the crowds. Occasionally some of these run off into the shadows causing havoic and slaughter here and there, but no worries. The Vigils don't bother to do down into the bowls of the ship unless necessary or in dire need of workers.
Each Ship also has it's own Garrison of troops and police plus the private armies of the Ecclestum and the nobles. These means anyone wishing to enter much less take a citadel class ship should pack some heat.