Crystal Cavern is a track from Bloons Tower Defense 6. It is rated Beginner. It is unlocked after beating Grassy Glade. It consists of bloons going around a few crystals, going up to the ceiling, then going around more crystals.
This was another defense. It was forged by the Genesis Bloon as a perfect array of crystals.
Laser and Energy Attacks bounce off the crystals. Placing a Ray of Doom on any crystal turns it into a Ray of Apocalypse, a blue beam of bloon death that utterly destroys bloons.