Well this place is VERY hidden in the city center. These are the remains of what once was the east wall of the former convent, which was built at the beginning of the 15th century. They demolished the building in 1583, but the wall remained. Years went by and in 1994 they broke down the new buildings on top of it and it came to light again. Now you can find it by entering the gate to the Algemene Rekenkamer at Lange Voorhout 8 in Den Haag and walk all the way to the back. Pretty cool that this wall is so old and stil here.