A tower defence game with all the trimmings, and ludicrous enemies to fend off.
Tower defense is a surprisingly under-represented genre on the Mac. There are some stalwarts every gamer should play, such as the excellent Defender's Quest, as well as the Anomaly and Cubemen series'. It's a vastly different story on iOS, of course, where that platform is over-saturated with examples of tower defence games, and indeed, which has served as something of an evolution chamber for it, with various features like hero units and upgradeable towers added over the years to the basic "plant down some towers to defend against waves of enemies" template. And from within this smelting pot of ideas comes OTTTTD (Over-the-top tower defense).