• In order to create a fortress, you will need 5 fully upgraded castles that are close to each other and have researched the new technology "fortress construction”.
• You will have to set one of these castles as the fortress center in its keep. This will mark the fortress’s location on the map. (Keep / create fortress center / create button)
• After that you will have to link the 4 other needed castles to the fortress center (tap the keep / expand the fortress center / create button)
• The distance from the fortress center to the bound castles must not be larger than 3 fields.
• You can create the fortress, once you have bound 4 of your castles to the fortress center. (tap the keep again / expand the fortress center / create button)

In order to demonstrate the creation of the fortress a bit more clearly, we created an animated graphic for you: