There are 9 different Global Defense: Zombie War Turrets and not all of them are just for attacking. Some effect the status of zombies while Radar will extend the fighting range of all turrets. Since you can unlocks the strongest turrets at the beginning of the game it’s recommended to do just that so you don’t waste points on things that will become obsolete later on. This will also make progression much faster. Placing units in areas that are not directly next to enemy paths will keep them from being damaged by exploding zombies.
More info at the Global Defense: Zombie War Guide Wiki
Gatling – Has a super fast rate of fire, but becomes worthless very early. You should avoid this completely and focus on ones that are better.
Frost – You will use this all the way up until the last level. It’s one of the cheapest to upgrade and should be done earlier rather than later. Cost little to place and and increase strength in battle. Should be place so enemies will always suffer from the slow effect by having multiple on the field.
Missile – Does good at first but then becomes obsolete early in the game.
Flamethrower – Does good at first but then becomes obsolete early in the game. Attacks groups and not just single targets.
Radar – Placing units in the range of this turret will give them additional distance for attacking enemies. Not so useful in the beginning you will eventually need this for weapons that have a short distance of attack.
Cannon – When this guy hits the zombies it will damage everyone in the blast radius not just a single target like others. Can help wear down groups, but should be invested in after Tesla is maxed out.
Anti-Aircraft – This one looks good on paper, but is almost completely useless when put side by side with the Tesla. Can help remove planes and helicopters when other weapons are targeting ground units as it only target flying zombie.
Tesla – The strongest thing in your arsenal and the most expensive to upgrade. You should unlock this right away and then invest in extra starting money so you can place it at the beginning of battle. You find out around level 30 the only thing really working is the Tesla which will be used until the very last level.
Shockwave – This one is a must and slows down zombies dead in their tracks for brief intervals. The more you upgrade this in battle the more times it will slow enemies before their out of range. Should be used side by side with Frost for maximum slowness.
Space Laser – Shoots a wide blue laser down from space. Invest in this when you have extra to spend. Great for free attacks to help out on difficult waves. To use hold your finger on the icon, then drag to desired location. After place you can move it around slowly by holding you finger in the diction you want it to go.
Shockwave Bomb – Drops a stunning bomb that will temporarily stop moving enemies. To use hold your finger on the icon, then drag to desired location.
Carpet Bombing – Will drop bombs along a straight path from left to right that cause damage. Not super useful, but is free and every little bit help later on.