The ogre cave is located below Dwarftown and is reached through the second staircase (located southeast of Thrundarr's chamber), which appears after activating the third of Thrundarr's quests. DL is one level higher than Dwarftown; consequently, if Dwarftown is D:9 then ogre cave is a corrupting area.
Initially populated by various ogres. Might have a tension room of non-ogre monsters, occasionally with ogres generated in it.
The important thing is that level does not generate random monsters, thus it often serves as safe place to store extra stuff or as a place for other activities like mining for ore or stone giants. Ruun can be led into the level and left there if he becomes hostile, e.g. if the player wishes to convert the city's altar.
Apparently ogres (of all kinds) from this location cannot be damaged by melee attacks from other monsters — this is intentional behaviour. Consequently, clearing the cave with pet is next to impossible, and the player can use the ogres to slaughter undesired inhabitants of Dwarftown (without most of the residents being able to seriously hurt the ogres). Glowing balls attack from Ruun (or emperor liches, etc.) will have full effect, however.
Thrundarr's third quest is to clear Ogre cave.