Star

From Orson Scott Card Wiki

A star is an obstacle found in the Battle Room that soldiers are forced to maneuver around. For any given battle, there might be zero or many stars, clustered or evenly spaced. Most armies (and their commanders) found stars to be nothing more than annoyances, since their locations were random and didn't necessarily work well with their preplanned formations. Some commanders, most notably Ender Wiggin, used stars to their advantage by having their men use them as makeshift bunkers that they could use as cover while firing on the exposed enemy troops.