Left Mouse Button:
Click on the desired turret type in the top right corner and click again on the game area in the spot you would like to place the turret.
-Select an existing turret to see it's range (The red dotted circle).
-Upgrade your existing turrets for greater effectiveness.
-Force the Xinos to walk farther then they normally would by building maze-like paths through the turrets.
-Use turret combinations to maximize their effect.
-Watch out for waves of flying Xinos that move right overtop of your placed defenses.