After you’ve attacked and captured an enemy wall or defensive structure, it will appear grayed out. That structure can’t be salvaged, and nothing new can be built in its land square, until the captured defense has crumbled and disappeared. Normally a defensive structure will crumble 10 hours after it has been captured; this gives the defender time to retake the land without having to rebuild the defense. About 20% of the time, a captured defense will crumble much more quickly, in less than a minute. After an enemy defense in a captured land square has crumbled, you can build your own defense in that square if you’d like.
When a defensive structure is captured, a timer circle will appear on it, indicating how long until the structure crumbles.