All the following applies to the Fortress, Temple and Citadel: 

Battles in the fortress are automatically calculated by the server. Members of the guild which owns a fortress can reinforce their armies directly into a fortress from anywhere. The reinforced armies will be stationed in the fortress based on their arrival time. 

Whenever an attacker's army arrives the fortress, the system will pick one army from the queue of the defending armies based on the army's arrival time, automatically run the battle between these two armies and then move on to the next defending army until all units of the attacker's army are killed. 

After the battle/s, both the attacker and defender/s will get a battle report. 

Moreover, w hen your guild has conquered a fortress all other incoming attacks now automatically turn into reinforcements. Be careful though, if your guild loses a fortress to an enemy faction and another guild from your faction takes it back. Any reinforcements sent before the fortresses was lost to the enemy, now turn into attacks.