Landguard Fort is a fort at the mouth of the River Orwell outside Felixstowe, Suffolk, designed to guard the mouth of the river. It is now managed by the charity English Heritage and is open to the public.
Wikipedia https://en.wikipedia.org/wiki/Landguard_Fort
Official Website http://www.landguard.com/
Coordinates 51°56'18.488" N 1°19'14.942" E