Your army or your city must be within the attack range of the target in order to initiate an attack.
The attack range can be increased by researching the technology 'Attack Range' in the Academy.
You can only attack 1 target at a time.
Once your army arrives at the target, you have a short period of time to start the battle. If you missed it, the battle will be cancelled and your army will retreat to where it came from.
After you attacked a city,  your surviving units will return home after the attack.
After you attacked an army, your surviving units will return to the location where it came from, may it be the player's city or the nearby tile the army was stationed before the attack.
You must be part of a guild in order to attack a Temple, Citadel or Fortress.
You can attack any player in the world, regardless it's an enemy or an ally. But be aware, attacking a target in an enemy faction earns you honor; but attacking a player in your own faction causes hatreds. It's your own decision to make, but be aware of the potential consequences.