As soon as I saw it, I had a great idea for a game. It's pretty simple too... Imagine a sort of hybrid of Angry Birds and Defender of the Crown (https://youtu.be/Af0vFi4sSzw?t=272). Where you have to build up your castle and then knock down your opponents.
You already have pretty good walls, rocks, brickwork, etc. So you could use that for the construction phase. Then you'd need to add a variety of weapons; Ballistas, Trebuchets, Cannon, etc. Each one would destroy a few blocks in a certain pattern. Maybe you can unlock/upgrade weapons as you proceed. You'd also need some kind of physics to detect when blocks are unsupported and allow them to fall & destroy themselves.
I'm envisioning a simple single player mode where you take it in turns defending and attacking castles and then an MP mode where 2 or more players can defend and attack each other. You'd need some kind of replication system to report which blocks are created or destroyed during the game, but that shouldn't be too hard for you.
Let me know if you like my idea.