All units taking part defending your fortress (no matter if still alive or killed) are lost, regardless of whether the attack was successful or not. In this case, new units must be trained to restore your defensive power.
In addition, the attacker steals parts of your wood and stone supplies.
If a fortress was successfully attacked, the so-called protection mode comes into effect. This ensures that a fortress is protected from attack for 24 hours.
Resources stored in the secret storage cannot be stolen.
An overview of the last fortress battles is stored in your mail. A button can be used there to start a counterattack.