A dungeon entrance can be built in the Centrepiece space of the Garden and Formal Garden in a player-owned house. It requires 70 Construction to build and when built, it gives 500 experience.
It is, as the name suggests, used as an entrance to a player's Dungeon. It can only be used to access the dungeon if the player has a stairway conjunction built below the garden. It can be accessed during challenge mode, which can be triggered by activating a lever in a throne room.
It should be noted that if you are looking to build a cheaper entrance into the dungeons you may consider using the trapdoor in a throne room instead, assuming you already have a throne room built. This also requires an oubliette to be built underneath. This is also an option for people who are looking to trade one garden space for a room they may already have.