with large murals of poetry, part of a wall poem project active from 1992, and still ongoing. Fortifications At the strategically important junction of the two arms of the Oude Rijn 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. Churches The chief of Leiden's numerous churches are the Hooglandse Kerk (or the church of St Pancras, built in the 15th century and containing a monument to Pieter Adriaansz. van der Werff) and the Pieterskerk (church of St Peter (1315)) with monuments to Scaliger, Boerhaave and other famous scholars. From a historical perspective the Marekerk is interesting too. Arent van 's Gravesande designed that church in 1639. Other fine examples of his work in Leiden are in the Stedelijk Museum De Lakenhal (the municipal museum of fine arts), and the Bibliotheca Thysiana. The growing city needed another church and the Marekerk was the first Protestant church to be built in Leiden (and in Holland) after the Reformation. It is an example of Dutch Classicism. In the drawings by Van 's Gravesande the pulpit is the centrepiece of the church. The pulpit is modelled after the one in the Nieuwe Kerk at Haarlem (designed by Jacob van Campen). The building was first used in 1650, and is still in use. The Heilige Lodewijkkerk is first catholic church in Leiden that was built after the Reformation. This church was given to the Catholics after the gunpowder explosion in 1807, which killed 150 inhabitants and destroyed a large part of the city centre. The 'Waalse Kerk' (Breestraat 63) was originally part of the Katharina Hospital. In 1584 it became the church of Protestant refugees from the Southern Netherlands (Brugge) and France. Later churches in the centre include the St. Joseph in expressionistic style. University buildings The