Awesome game! It's simple yet extremely fun, also, the music is on point. My only suggestion would be to perhaps prevent the player from boxing the enemy in, but it does add another layer of strategy to the game. Great job!
I'm no programming maestro either, but given that you're using a tilemap-based system, before placing a block, you could run the preexisting pathfinding algorithm (or one from the Asset store) with the tile added in code (w/o actually rendering the tile) and if there is no path, then you can prevent the tile from being placed.
If you want a simpler method, you're already doing a check after the block has been placed, so if that check returns false, simply undo the placement. I might also consider doing tile-based movement (from tile to tile) as this would make pathfinding a lot easier for you. Just my two cents =)