Usage patterns
〈場所〉に要塞を築くto build a fortress at a location
They built a fortress on top of the mountain.
要塞を攻めるto attack a fortress
The enemy forces attacked the fortress.
要塞が陥落するfor a fortress to fall (be captured)
After a long siege, the fortress fell.
〈場所〉を要塞化するto turn a place into a fortress
There is a plan to turn this island into a fortress.