One may find Outer domains as well as Underground domains called Barracks in the Dungeon.

Outer domains can be found both during exploration of the kingdom and exploration of the Dungeon and will be seen on both cells.

Barracks can be found only during the exploration of the Dungeon.

To send explorers to the Dungeon, open your Expeditionary Corps and check Dungeon next to the coordinates of the cell

Missions cannot be sent from other cities. Underground missions can be sent only from Pits, Mines, and Capitals.

That is why when trying to send a mission to the Dungeon from an inappropriate estate, you will see a notification window:

There's a penalty on the duration of all underground missions in the Dungeon: +25%.

If you send an underground mission from your capital, you will have an additional penalty on the duration of the missions, which is +15 min.

Penalty "+25%" can be temporarily canceled with the help of a Tournament service