Apparently clowns can move from inside a solid wall tile to an adjacent empty tile - FPS hit and then fireball spam at the !!miner!!. Smoothing and carving fortifications around them had peculiar results that seemed to indicate a creature inside a wall cannot see/ be seen orthagonally but
might be able to see/ be seen diagonally - test abandonned as a failure due to no fireball spam though.
Test-collapsing the tile has to be abondonned as well since the 3x3 section must be dropped whole to prevent escape and it borders on the magma sea on one side. I just don't have the population for another major sacrifice of dwarves to gain access to any more of the magma sea floor for construction.
You should see if the presence of the trapped demons prevents any more from spawning on the map edges of hell like wildlife. If it does, you can colonize in complete peace.
This is an interesting thought. I'm too invested in this fort's mega-projects right now to risk breaching the circus, and my military is dropping like flies from old age but that will have to be investigated as an end-game scenario after the BLACK H.A.L.O. is completed. I do have about 1k of steel bars stockpiled.
For now they are trapped behind walls covered with un-inspired engravings; This is an exceptional image of a muck root.