A "castra" refers to a Roman military camp or fortification. These structures were strategically built to house soldiers and protect them during military campaigns. Typically, a castra was designed in a rectangular shape, featuring a surrounding wall, gates, and internal streets. The layout allowed for efficient organization and defense. The term "castra" is derived from the Latin word for camp, and it played a crucial role in the expansion of the Roman Empire. Each castra could accommodate a legion, which consisted of around 5,000 soldiers. The design and construction of these camps influenced later military architecture throughout history.