Outer works refer to the external defenses or fortifications of a military or strategic installation. Synonyms for outer works include boundary walls, exterior ramparts, perimeter fortifications and external defenses. These works are designed to keep intruders from gaining access to key installations such as military bases, prisons and airports. The outer works may include physical barriers such as ditches or walls, as well as surveillance systems and other forms of security measures. Foreign invaders and rebel forces pose a constant threat to such sites, and therefore, outer works play a crucial role in securing them. Synonyms for outer works serve as suitable descriptions for any form of perimeter protection around an area of strategic importance.