Bloon Citadel

Bloon Citadel is an expert track unlocked at rank 40. There's a small pool of water at the bottom, which can't really reach anything. There are 3 starts and exits, and you can likely tell where the paths are. However, the bloons can also hop onto other paths and back on again, making for there to be a lot a possiblities.

Backstory
This was an abandoned bloon castle, built near the beginning for the 5th war and abandoned for unknown reasons sometime after the end of the 5th war. Some monkeys tried to loot the castle, but they couldn't find the throne room, where most of the castles's king's treasure was likely kept. Legend has it that it was sealed up, and unless you find 3 special things from 3 special blimps, you can access the throne room. Inside, will be a lot of treasure. Say, what are those key-like indents in the middle of the track for?