You can place the turret by selecitng "Normal" and then selecting somewhere on the map. In addition, if you select the placed tower (by clicking directly on top of it) you can see the range of the tower and hit "U" on the keyboard to upgrade the tower. I've given you 500, so you can play around a little with the mechanic.
Select from a variety of towers, including: Basic, Shotgun, Poison, Minigun, Flame and Sniper. Use the mouse to place the towers, based on the amount on gold you have. As an added bonus, use special abilities to kill your enemies when things get a little tough