Cages, like chains, hold potential allies. Cages come in groups containing a variety of monsters. They can be opened with a cage key
cage room
, which will be found on the level as part of a machine. The cage key can only be used once or, sometimes, twice, so you must pick just one or two allies to take with you.
A Wand of Beckoning or a Wand of Teleportation will allow you to free an ally without a key, and a Wand of Plenty will produce a duplicate that is not caged, but is not your ally either!