This is a structure in the centre of Leiden.
The below is from the Holland.com website.
The Burcht van Leiden is a 20 metres high citadel in Leiden’s centre that served to defend the city. The construction of the hill is thought to have started as early as the 9th century with turf and clay. The citadel has walls that are six metres high and many embrasures. The citadel offered an excellent view of the city and its surroundings so that any approaching threats could be detected and a counter-attack mounted.
These days the citadel is a much-loved attraction in Leiden’s inner city. The entry is free and many visitors consider it an exciting adventure to climb the old citadel. And the view from the top is still as impressive as it was in the past.
Wikipedia said the following too.
At the strategically important junction of the two arms of the Old Rhine stands the old castle de Burcht, a circular tower built on an earthen mound. The mound probably was a refuge against high water before a small wooden fortress was built on top of it in the 11th century. The citadel is a so-called motte-and-bailey castle. Of Leiden’s old city gates only two are left, the Zijlpoort and the Morspoort, both dating from the end of the 17th century. Apart from one small watch tower on the Singel nothing is left of the town’s city walls. Another former fortification is the Gravensteen. Built as a fortress in the 13th century it has since served as house, library and prison. Presently it is one of the University’s buildings.
We had been previously but I hadn’t gone up the stairs the structure.
But these are images from inside by me
You them go up some more stairs inside here to walk around the wall, a bit like Derry city and York.
At the entrance/exit there was this sign.