Laser Tower - A relatively powerful building that is great for taking out groups of weak monsters. It hits wide areas within a short range. Re-routing monsters from different areas near the laser tower by using blocks would be an effective way since it can take out large numbers of monsters at one time but only in a short range.
Tesla Tower - This tower has the 2nd longest recharge time next to the railgun but makes up for it with the power to take down the all-mighty DAVE. This tower is the opposite of the laser tower taking on strong enemies in small groups. Teslas can be placed anywhere near things you dont want to be destroyed the most such as Storage Silo's or your town hall.
Sniper Tower - Sniper towers are relatively weak compared to the first two primary towers mentioned before but can be very helpful. Putting them behind your first or even second line of defense can certainly do some damage or can be used as a "last defense". They have great range and can be effective towards the center of your field. Once upgraded to level 4 and above, they deal a nice amount of damage and can be used efficiently as long as they are backed up by something close-range and powerful, such as a cannon tower or laser tower.
Cannon Tower - The strategy that can be effective with this tower is similar to the laser tower because it has a splash damage. It is useful when around sniper towers as they make a very good combo, one weaking the monsters and one using splash damage to finish them off.
Railgun - A very powerful building, a good technique is to put two parallel lines of blocks and the rail gun at the end of the blocks, which shoots a infinite line of fire and can damage large groups of ground monsters easily. It has wide range also which is also helpful. The only downfall of this is its long reload time. Never use it on its own.
Aerial Defense Tower - When facing aerial enemies, such as Teratorns, Zafreeti, and Fomor, these towers can save your whole yard. They can kill a level 1 Teratorn in less than 2 seconds and can dispatch whole attacks of flying monsters when backed up. These towers hit air monsters with splash damage (hit more than one monster at once). Their only down fall is that they cannot fire at ground monsters, which, unless backed up, gives the attacker of your base the oppurtunity to destroy it using ground monsters then perhaps fling in a couple of Zafreeti's and Teratorns to finish the base off. At higher levels they are one of the most important towers in your yard as they don't shoot at ground monsters. This means they will kill Fomor and Zafreetis which will be supporting ground attackers. It can also attack Loot-o-Tron 5000 looting your base.