An 11th-century shell keep sitting on a grassy mound in the centre of Leiden, so tucked among the surrounding buildings that you can walk right past it without realising there is a castle above your head. Climb the steps to the top and you are rewarded with a panoramic view over the rooftops, canals, and church spires of the old city — one of the best viewpoints in Leiden.
The castle is a public park, open daily, free to enter. The circular wall at the top dates from around 1150, making it one of the oldest structures in the city. It feels oddly peaceful up here, above the bustle of the streets below. The entrance is through a gate on Burgsteeg, near the junction of the two branches of the Rhine.