This is St. Leonard's Tower, near West Malling, Kent. It was built between 1077 and 1108- most probably by Gundulph, Bishop of Rochester. No-one's quite sure what it's for. It's too small to be a castle, too big to be a bell tower and not fancy enough to be bishop's palace. It was probably some sort of administrative centre.
No matter, it's Romanesque and I love it.