The Priory Church is one of London's oldest churches. This whole area was originally part of the medieval St John's Priory and is now associated with the revived Order of St John. The walled garden, planted with medicinal herbs and flowers, was built as a memorial to St John's workers who died during the world wars. Sadly the church can only be visited on a guided tour, along with St John's Gate.
If the somewhat boxy street-level church doesn't seem like it ever belonged to a medieval priory, that's because it didn't. The real treasure lies beneath, where the nave of the original church has been preserved as a darkened crypt. Built in the 1380s in the Norman Romanesque style, it's one of the oldest buildings in London. Inside there's a sturdy alabaster effigy of a Castilian knight (1575) and a battered monument portraying the last prior, Sir William Weston, as a decaying body in a shroud (a memento mori designed to remind viewers of their own mortality).
The nave once abutted a large circular chancel that was demolished following the dissolution of the priory. Outside, the outline of the original church has been traced onto the square.