Airstrike

Call in reinforcements that will drop heavy ordnance to the specified location. This deals devastating area damage, capable of instantly killing hardened Creeps. This damages both land and air targets. Since friendly units and heroes are not affected by collateral damage, this is best used in a bottleneck where enemy forces are engaging your soldiers since the targets will be stopped on their tracks, allowing you to kill more in one sortie.

All upgrades are unlocked in a linear manner; you have to purchase the preceding upgrade before you can unlock the next one.