Great Idea
The cannon angle needs faster adjustment. Either click and hold, manual input number value, or a slider of some sort. Killing the cannons is basically the goal, as the castle can take a crazy beating. You may want to change that. As it stands, its a replication of the old artillery game, only with 2 arty pieces and a second more difficult way to win (the king). Balance the issue I mentioned, and fix the power and angle adjustment, and then look at other options, like the features you would find in worms.