Primal Defender is a first eraNaturetower. It has the ability to attack ground and the ability to attack flying creatures, but these abilities are mutually exclusive. Enabling the air attack causes the building to be unable to attack ground units, and vice-versa, requiring some micro-management or placement of multiple buildings to defend against both types of attack.
One issue with Earth Strike is that it requires line-of-sight to attack its target. If another tower or entity blocks the projectile's path to the target, the projectile will not fire. Placing towers in strategic formations like a 'V' to funnel enemies into a kill zone can help prevent towers from being marginalized. Likewise, in PvP, a player should keep an eye out for poor Tower formations that can be used to reduce incoming damage.