The interior of the Chester Ridge Ruins is filled with fire, shadow, and sinister enemies!
It is also filled with this initial fire arena:
To make this work in roll20, I first made the “tame” map the background for the map on the map layer, then made a rollable table with every other form of the map. Once it was set up, I moved the rollable table “to back” to hide it, and once the battle started moved the tame layer to back and every round would randomize the table to determine what floor burn would be in effect. In the center of the platform I had a boss with high health that shot very powerful beams of fire in any two cardinal directions each round, making moving on an off the platforms essential for player survival.