In 1672-'73 the 15th century city walls were replaced by a new wall as part of the Oude Hollandse Waterlinie. Johan Maurits van Nassau made a design according to the New Dutch system, but of this plan with eight bastions, only half was realized, namely the Utrecht front (south side) with the bastions Rozeboom and Draaierschans and the 's-Gravelandse front (east side ) with the bastions Bakkerschans and Nieuw Achtkant.
These two latter bastions form an island in the Vecht. The shape of the other four unfinished bastions are still recognizable in the street pattern. At the end of the 17th century, Weesp became a permanent part of the Dutch Waterline. The bastions were improved and further east the Fort Uitermeer came to lie.
In the framework of the New Dutch Waterline, new reinforcements followed from 1845 and in 1892 Weesp became part of the Defense Line of Amsterdam.
On the Vechteiland in 1859-'61 the Fort Ossenmarkt (Ossenmarkt 44) arose as a round tower fort of two storeys - one of which is largely underground - with a semi-circular stairwell. In 1876, the surrounding canal was filled up and a heavy covering wall was built on the east side. These changes were reversed in 1983-'85. In 1991-'92 an interior restoration was carried out and a wooden access bridge was installed. Near the fort are a casemate (1870), a wooden artillery shed (1877) and concrete defenses (1939-40).
