Nice
I really don't see Tivaelydoc's problem with this game. I think the physics sre fine, considering you are launching giant boulders (not rubber balls) at a castle made from the same stuff, and having the ball bounce (rather than crash through) to score combos. It's fairly simple: you added a bit of fantasy to the mix.
However, the game is not without its flaws. Personally, I think the aiming could have been improved a bit, and possibly have the guards stronger. As it is, you are forced to choose +1 ammo every round until you get the hang of the aiming. By that time, however, you are being overwhelmed by enemies, with only one soldier, and you don't have enough rocks to crush them all.
Here's an interesting feature, that would make the game slightly better, allow the rocks you throw to roll a little as they land. All you would need to do is add a slightly less constrained friction system and give the boulder a rolling animation. This would let you take out more than one soldier at a time with your rocks.
I also have a suggestion for an unlockable mini-game, bowling. You launch the rock into the air, or hurl it across the ground to knock over a tower of soldiers (or pins, if you prefer) on the other side of the field.
Whichever way you go, or whatever you do, I am looking forward to a sequel.