A small stone or timber structure built beside a river or harbour mouth to house the end of a defensive chain, or the mechanism to raise and lower a defensive chain. Such chains were laid bank to bank across a river or inlet so that in normal circumstances they rested on the riverbed or sea bed. At times of trouble they could be lifted to run more or less along the waterline, thus barring access to hostile ships. The towers were usually strongly built so that they could be defended against attacks from landing parties set ashore to unblock the navigation. Most chain towers date to the 15th and 16th centuries ad.