Certain defenses have a priority of enemies it targets first if said enemy is within the defense's range. This priority may be changed in the Spellbook. The different priorities are as follows:
This priority focuses on the higher-frequency, weaker enemies.
This priority focuses on the lower-frequency, more dangerous enemies.
This priority focuses on air enemies over ground enemies.
This priority focuses on enemies with larger health pools.
This information was first available in this forum post: https://forums.dungeondefenders.com/entry/159382-october-27th-episode-1-the-lycans-keep-patch-21/