Monster Nests are alternate spawn points for monsters, letting them attack from the middle of the screen rather than the border. The nests can be destroyed by bombing them or targeting them with towers, usually having a few thousand HP and several hundred armor. Destroyed nests cannot be built over, but destroying a nest may let you build on pathway if it is no longer blocking monsters. Some nests may be invincible, acting as a permenant spawning point for the battle.
Monster nests are fairly common, existing on nearly half of the fields in Gemcraft Chapter 2.
Current list of levels with monster nests(please contribute to this list):