A chain tower is a small structure built beside a river or harbour to house the end of a defensive chain or the mechanism for raising and lowering it. Harbour chains were intended to protect estuaries, harbours, and river mouths from attack from the sea; they were laid from bank to bank and would normally rest on the bottom of the channel, being raised to the waterline when under threat. One end of the chain would always have a tower or building to house the lifting mechanism; the other end could have a similar structure, or a simpler means of fastening the end of the chain. Chain towers were usually, but not always, strongly constructed and capable of being defended and these were built of stone; the others, now lost, were of timber, or earth revetments. They were sited near the water's edge. Accommodation in the chain tower, if provided, was only for short-term use by the chain operators and defensive garrison. There was great variation in design, but the main components were the chain, with either two towers, or one tower with another structure at the far end of the chain, and there may be a dry ditches or moats on the landward side.
Sites that may be confused with this class are blockhouses which are considered as a separate class of monument; there was some overlap in function, as at Dartmouth Castle, Devon, and the Round Tower at Portsmouth, Hampshire, both of which functioned as blockhouses and chain towers. Chain towers will generally be nearer the water, and less defensible, whilst blockhouses may be raised higher where possible and be more obviously defensive; a chain tower may sometimes be protected by a blockhouse.
Chain towers were designed solely to protect a maritime entrance from attack from the sea by barring access to hostile ships by means of the chain. The first certain example of a tower being built for a chain is in 1457 at Fowey, Cornwall (History of the King's Works 1982. vol 4, 593), and a late example was at Gillingham, Kent, in 1667 (Saunders 1985); most were constructed in the late 16th century.