A temple is a structure devoted to an object of worship, any Sphere-aligned being such as a deity, megabeast, or titan. They are located in Human towns and Goblin fortresses, and the object of worship will be consistent with the Sphere or Spheres the Civilization is aligned to. Priests can be found in temples, and will allow you to join the local sect of their religion with the conversation topic 'Service'.
On the first joining a sect dedicated to a particular object of worship, that entity will be added to the character's record. An adventurer can then converse() anywhere with any deity object of worship. Though the deity's only response is silence, repeated conversations can change the deity's object of worship status from 'dubious' to 'casual', 'object', 'devoted' and 'ardent'.
It's unknown whether conversing with other objects of worship can increase their worship status; megabeasts tend to be uniformly hostile, though Titans may be flagged 'benign'Verify.
Occasionally you will find a human temple with a large pool of magma at the bottom.
Temples can have basements, sometimes extensive catacombs which may be crawling with enemies. These range from the typical kobolds and goblins, to the occasional lurking Mummy (with assorted Skeletal minions).