Players can send out their units, defeat the monsters that reside on it, and take control of the tile. Land that is under player control can be taken by someone else, but its Field HP must be reduced to zero first.
Land can be captured only if the empty tile is next to land that is under you or an alliance member's control. Players can still attack land or ruins that are not connected to collect rewards, but the land will not be captured.
Captured land can be turned into Open Land again with a timed penalty. This feature is useful if you need to relocate land to take an offensive or defensive position. From the Map screen, tap the desired tile, tap Details, then Abandon to dispose the land.