Ground on which each side has liberty of movement is open ground. 572

Ground which forms the key to three contiguous states, 573 so that he who occupies it first has most of the Empire at his command, 574 is a ground of intersecting highways. 575

When an army has penetrated into the heart of a hostile country, leaving a number of fortified cities in its rear, 576 it is serious ground. 577

Mountain forests, 578 rugged steeps, marshes and fens⁠—all country that is hard to traverse: this is difficult ground. 579