Your Units will return to your Catacombs without attacking a hostile Castle or Hamlet in the following cases:
1. Your opponent activates a Protection item over his or her Castle.
2. Your opponent relocates their Castle to another location on the Map.
3. A targeted Hamlet becomes depleted before the dispatched Units arrive to capture it.
4. Your opponent loses control over the Hamlet your Units were sent to attack. It is impossible to attack a vacant Hamlet.
In order to minimize damages, some players choose to vacate their Hamlets before incoming Attacks arrive.