The Monster Garden:
A festering growth of vile magical plants, the roots of which stretch all the way down to hell. Every so often, the womb pods of the plants will vomit out a number of vile creatures. The creatures evolve and change in small ways over time, making every harvest different than the last. War is the only constant.
When they reach maturity ( a matter of hours) the garden monsters will set out in force to cross the lake in Arena cave, making war on the dwarfs and gear of the fortress there. Many battles are fought on the surface of the lake.
A force comprised of a lost dwarf clan and their magical robot soldiers (called gear), keeps vigil in this cave. They are sworn to keeping the spawn (their name for the creatures that come down the stairs at the far eastern side of the lake) in check and forever contained. They are unaware of the true nature of the garden and fight with a grim fatalism, accepting that some day they will be overcome by the spawn.
There is a courtyard deep within the keep where the dwarfs grow the souls of the gear on glorious magical ghost trees.
Adventure hook: explore/destroy the garden.