TurretWars MP is a multi-player strategy game for the Mac. In this game, you control a cannon turret and have to destroy your enemies before they destroy you. Even though the game was designed for you to play against friends, you can also play against computer opponents. And there are different difficulty levels, which makes it easy to find the one that is challenging for you, but not too challenging.
When you start playing, you will see your opponents on a map. When you shoot a projectile, you will see its trajectory from a 1st person view, which makes it easy to spot your enemies there. Once you know where an enemy is, you can fine tune your shots. The map is filled with hills, so some times, you will have to shoot a fast and low trajectory shot, while others, you will have to shoot them slow and high, calculating when the shells will fall. You will damage enemies even without direct shots, but the direct ones inflict more damage.
Finally, this is an interesting multiplayer game. It is not an easy game to play at first, since there are lots of controls, and the only help you get is in the form of a PDF file. But if you keep on playing it, you can figure it out eventually.