What is "achterdam"

The Achterdam is a red light district in the Dutch city of Alkmaar, 30 km North of Amsterdam. It is the only place in Alkmaar where window prostitution is permitted. It is situated about 10 minutes walk from the Alkmaar train station. It is a 150-metre-long street with window prostitution on both sides. The area has about 100 windows with rooms. Mainly Eastern-Europe women work there. The rate paid by prostitutes to rent rooms is 125 Euros per day, with prostitutes applying daily for rooms on a first come, first served basis.