Dungeons consist of a network of rooms, containing various types of monsters based on the theme of the dungeon. The last room will contain one or more boss enemies, which may drop special items.
To reach a dungeon, it is necessary to kill certain enemies, which will randomly or systematically open the entrance to that dungeon. Any player can enter to fight the monsters inside, usually by clicking “Enter” or using the interact key when standing on an entrance.
All portals stay open for 30 seconds, except for the Locked Wine Cellar Portal (open for two minutes) and the The Shatters Portal (open for 70 seconds).
You can find a complete list of dungeons [here].