Ridge Wars.
This map is pretty basic, except that it has two ridges running next to the castles. From here, your enemy's artillery can bombard half of your base, or you can do the same to your enemy. This creates interesting battles for the ridges, as each has only one access point. While this map is intended to be multiplayer, it could work as a singleplayer vs. the AI. However, the AI will not take advantage of the ridges, so it isn't much of a fight. Hope you enjoy it!