I like the central tention in the game, the limited selection of blocks forcing a diffrent solution on every map.
I like the diffrent cool looking castles and seeing if my plan worked.
I would realy like:
An undo button for when I put a block in the wrong place.
I would quite like:
A redo button, to run the same map with the same blocks again, to see if I can do better.
A score representing total damge. (With more points lost if the keep is attacked).
Maybe would be cool:
A gate house block. It would sit elivated either side by stone or battlements. The attacker path would not be impacted by it, but something could rain down on them as they passed underneath. I think it would look cool.