A vast, labyrinthine castle floats in deep space. It's long abandoned by whatever agency constructed it and partially in ruin, but still functional. Runic engines thousands of years old hum, providing heat and gravity and keeping the air in. Managed ecosystems run wild: the roots and leaves of strange plants are everywhere, wound in thick ropes or tightly-knit nets, and small animals originally meant, it's thought, to recycle debris scurry and scamper through grooves and cracks. Even after unknown ages, they still keep the air clean, secrete fresh water for the fountains, and sprout odd, edible fruit. Even a derelict, the castle could still support thousands.
Its architecture is thoroughly alien. Nothing is scaled for humans, nothing quite makes sense. Halls are cyclopean or miniature, passageways spread in mazes laid out by some unknown logic. Vast support structures jut through walls at odd angles as if added in as an afterthought. Parts seem fortified or intentionally designed to waylay attackers and be easily defensible, yet vast arches open directly to the vacuum in defiance of any notion of security. Most rooms and structures have no discernible purpose. Some are thought to be temples, others perhaps storage areas or gathering places, but it's impossible to be sure. There are statues and frescoes and other decorative elements, but they are all abstract. No depictions of the castle's builders, and nothing recognisable as writing, appears anywhere.
Small parts have been settled by different groups, and there is evidence this has happened before. Some work to restore segments of the castle best they can, others to uncover its secrets or to plunder its wonders. Treasures are found buried in the deep halls: jewels, metals, unmanufacturable materials, impossible machines, works of art entirely unlike any recognised style or school. Most of the castle is merely lonely, an enormous manufactured desert more forlorn than any natural wilderness, but there are dangers, too. It's difficult to say if they are traps, defence mechanisms, malfunctions, or merely misunderstood, but parts of its construction can kill; and though most of its inhabitants are no larger than a squirrel, and very few are anything other than curious, there are some that are predatory, territorial, or violent when threatened. Stretches of the castle appear impossible to map, or have been observed to change their shape. It's common policy not to send search parties to the deeper layers.
Somewhere inside there must be a power source — and it must be strong and enduring indeed — and a command deck, but none have been able to find them, or return if they have. Its useless to try and probe the castle from outside: beyond the parts directly exposed to space, something blocks out all transmissions. Neither have violent attempts to burrow through been successful. The castle moves to evade impacts, alters itself to section off invaders, and retaliates attacks with multicoloured fire.