Where the old Rhine reached the North Sea, near the modern town of Katwijk, the Romans built a string of forts, and Valkenburg was one of them. The Dutch Valkenburg, a village near Leiden, not to be confused with its namesake in Limburg, was the site of a fort and harbour called Praetorium Agrippinae, a base on the northern frontier of the empire that stood on the south bank of the river. Along this stretch the Rhine formed the border of the Roman province of Lower Germany, and the line of forts, watchtowers and roads along it is part of the Frontiers of the Roman Empire World Heritage listing.
Little shows above ground, since the Dutch landscape has always been busy burying things, but excavations here have turned up wooden structures preserved by waterlogged soil, including parts of the fort's timber walls, ships and personal items such as leather shoes and writing tablets. The Rhine itself later silted up and shifted, and the sea moved the coastline, so what was once a frontier river estuary is now a quiet lowland of canals, polders and dunes.
Riding in this part of Holland is flat, tidy and dominated by wind. The Rijndijk and its neighbouring roads run along old dikes, with water on both sides, and the air smells of cut grass and salt. In spring the nearby bulb fields around Lisse and Noordwijk are a carpet of tulips and hyacinths. Cyclists have priority everywhere, and cycle paths run alongside the roads, so look carefully at junctions. The Dutch enforce speed limits strictly, with cameras on many roads, and the weather can turn from sun to squalls within minutes.
Valkenburg is a small place, and the nearest bigger town is Rijnsburg, with Leiden a few kilometres away, where there are fuel stations, supermarkets and cafes. In Leiden you can see the Roman finds in the National Museum of Antiquities. Try fresh herring with onions, eaten from a stall by the canal, and a strong coffee with a slice of apple pie. The dunes at Katwijk, a short ride off, are a good place for a walk, and the best light comes in the evening.