How do the landmarks work?

Each landmark shows a countdown timer, when it reaches zero a so-called preparation stage starts. After 60 minutes the challenge stage starts which lasts for 3 hours. During this stage alliances that occopy territory bordering the landmark can attack it, and defend their control by sending more troops as defense. You do need to beat the landmark guardian force (135.000 T9s!) first. The alliance that held the most control gains the landmark until it is taken over during the next challenge stage.