Also termed seirō yagura 井楼櫓. A watch tower. Seirō were built from the Muromachi to the early Edo periods to spy on enemies, and sometimes also to shoot arrows or guns from. Early seirō were constructed with a simple wooden framework and no roof, but later versions often had a more solid structure with a tiled roof and thick walls. Heights varies from one to five storeys; each watch tower was build to meet particular local military requirements. The watch towers were set up inside or outside a castle, or atop the corners of stone castle walls. Some seirō were mobile versions fitted with four wheels; these were called kuruma seirō 車井楼. Others, kumiage seirō 組上げ井楼, were only assembled when required, and the height could be adjusted as necessary.