I have noticed that some players in Royal Revolt 2 did not set troops to defend their kingdom. That means when enemies are coming close to your castle gate, the troops you have unlocked like Warrior, Archer and Froster won’t go out to protect your castle.
To let your troops to fend off invasion, you must upgrade your Castle Guard on the one hand and add units you have unlocked to the slots. If you cannot find where the slots are, just open the Castle Guard, you cannot miss it.
The slot that each troop type occupies varies depending on what type you select. Make sure choose appropriate types according to your own strategy.