Rochester Castle contains one of the finest and best-preserved Norman keeps in England. Built in the early 12th century, the great tower stands 113 feet tall and dominates the town from its position above the River Medway. The walls are twelve feet thick at the base, which goes some way to explaining why the structure is still standing nearly 900 years later.
English Heritage manages the site, and the interior is open to visitors. The keep is largely a shell now — floors and internal fittings are long gone — but you can climb to the top for good views over the river and across to the cathedral. The grounds around the castle are a pleasant public park, and the combination of the ruins, the river, and the surrounding gardens makes this the most atmospheric spot in Rochester.