Beta Layer 3
By using Lunar Magic, you can insert a third layer in levels. If you insert layer 3 into levels with a tileset other then 1, 5, 9, D, or E, there will be what appears to be a cage in the level.
For this cage to actually work, you must insert sprite 88, as well as an autoscroll (2 or 4 work best). Note that the graphics for the sprite were never actually finished, thus causing it to look all glitchy. You can fix this, though. The sprite uses castle layer 3 graphics, and you can edit that graphics file and insert corrected graphics.
This uses the following sprites:
Cage: Sprite 88
Any of the autoscroll sprites (2 or 4 work best): E8