In the deck construction, dungeon cards are considered the free ones, and you can add only one dungeon in a deck. They don't count as part of a deck, which means that decks with the dungeon included have 56, 76, or 111 cards. Consider these cards as personal rule cards, with only their owner's effect.
Dungeon cards are shuffled into the deck like normal rule cards. On your turn during phase 2, you can play your dungeon from your hand. In the same turn, you can play a realm, a holding, and a dungeon.
A dungeon card can be removed in several different ways. One way is by using the cards meant to do that - Dungeon explosion (496/Prime), or you can attack the dungeon to remove the card.
Other players can attack dungeons by the same rules as attacking the realms. Opponents should have a "path" to the dungeon, and Unrazed realms protect the dungeon from attacks.
The realm in the same spot as the dungeon does not provide protection. If the realm with the dungeon in the same spot is discarded, the dungeon is discarded too. If the direct attack on the dungeon is successful, the card is discarded, and the attacker gets dungeon spoils. The defending player gets the dungeon spoils if the defender defeats the attacker. In either case, combat ends.
Some Dungeons have powers to hide champions, and champions placed in the dungeons are not considered "in play." Another player may add an identical champion to his pool, but when the champion leaves the dungeon, the identical champion "in play" should be discarded. Check who is hiding in the dungeons in order not to lose your champions.
There are alternative rules to play Dungeon cards. If you find this useful, ccheck out the Optional rules for more Strategy and Fun section.