Open Puzzle Continent on your mobile phone and [wait for loading] to enter the game. As shown below
After entering the game interface, click [Castle] to enter. As shown below
After entering the Fandi Castle interface, click [Upgrade] to enter. As shown below
After entering the upgrade details, click [Upgrade] again to enter. As shown below
Finally, just see that the castle has been successfully upgraded to get rewards. In this way we know how to upgrade the castle in Puzzle Continent. As shown below