why is everything so square and stacked in cubes
Maybe they are only destructible by the boss, like this castle is designed so that this time, players can't simply rush towards the boss if they wanted to. it's scripted to come out on its own after a certain amount of time. If players can destroy the blocks too then they will have to plow through them to reach the boss, any leftover blocks can be used as platforms.

Based on the screenshots, Menace can destroy the black/bones platforms/walls/ceilings too.
Another theory is that its the players job to reach the boss by destroying the purple cubes.
They cannot destroy the black cubes. Once they reach the boss, it gets up and proceeds to destroy any black and purple cubes in its path.