Reign determines how much land can be controlled outside of your home castle. The maximum amount of Reign is automatically increased over a set period of time, but can be accelerated by constructing the Heroic Statue. Taking control of an open tile will consume one point of Reign, while disposing it will regain one point as well.
Land, towers, and Castles have an innate "Field HP". This number determines how difficult it is to take control of a particular tile on a map. Land that has not been controlled by anyone will have 1 Field HP. Land that is controlled by another player will have 100 Field HP. Other tiles like Debris Ruins can have thousands of Field HP. The "Siege" parameter of your monsters will determine how much Field HP is drained for every successful attack.