Basilica

This special mission takes place on a new track. However, whenever you place a tower in one of the spots, it shrinks or grows to the size of that spot. Also, the spots are the only places you can have towers. Therefore, you can only have 8 towers at a time. You face Hard Difficulty, with the boss being the S.I.E.G.E on Round 100. The reward is $800 MM.

Trivia

 * This is a reference to Bubble Tanks Tower Defense 1's Basilica Track, where there is room for 8 towers.
 * Beating this gives you the achievement "Castle Defender", which is worth 125 AP.