Attacking units are used to do damage / destroy enemy units/buildings, to destroy enemy transports, or to immobilise enemy priests. When an attacking unit is placed, you can see its range by selecting (left-clicking) on it. The attacking unit will be able to attack enemy units/buildings within this range, and if it is an aviary, its produced flying attackers will not attack units outside of that range. Attacking units usually target the nearest target within its range, but they cannot target invisible units, nor can they attack whilst under the effect of paralysis.