Barking Abbey was one of the most powerful religious houses in medieval England. Founded in the 7th century by St Erkenwald for his sister Ethelburga, it survived Viking raids, royal disputes, and a thousand years of English politics before Henry VIII's dissolution in 1539 reduced it to rubble.
What remains is an open-air ruin in the centre of Barking: low walls of flint and rag-stone marking the floorplan of the abbey church, framed by the surviving 15th-century Curfew Tower (also called the Fire Bell Gate). It is the kind of site you can walk through in fifteen minutes or sit in for an hour. Information boards explain the layout. Free, always open, and immediately next to St Margaret's Church.