Reached iteration A6-18. the only thing I didn't like was that towards the end, if a spike formed in the middle, you where just kinda cooked if you got a tall piece. due to the placement of unusable tiles it was very difficult to get rid of anything in the centre 2 columns, making this issue more or less inevitable. some grace period after the polynomino enters the board, where it can go up a bit to get over spikes, and won't place, could fix this, but I don't know if that fits your vission for the game. clearing space did feel meaningful, and I mostly understood what was going on. only exceptions where that I have no idea what catalysts do, or how to use them, and I could've sworn one of the modefier thingies said it would let white tiles connect on diagonals, which it did not, but I could have been misreading that.