Dungeon Level 4 pushes the complex back toward natural underground terrain. Rough cave openings connect to worked rooms and passages, with patches of earth, moss or water interrupting the more regular stone construction. The level feels partially overtaken by the cavern around it, giving the lower dungeon a different rhythm from the cell-lined floor above.

For a GM, the mixed terrain creates flexible encounters. Narrow constructed approaches can funnel the party into broader caves, natural cover breaks sightlines and side rooms give defenders places to retreat or stage secondary objectives. In Beyond the Doomsday Door, this lower level helps the dungeon feel physically deep rather than simply repetitive. It can also be reused as a ruined underground temple whose foundations have opened into a living cave system.