City archaeologists suspect that the old walls they keep turning up during excavations around the Nieuwezijds Kolk are the remains of a 13th century castle, possibly that of Gijsbrecht van Aemstel, Amsterdam's founding father. It is true the finds are the oldest ever made in the city, and that the building was definitely a defensive work with corner towers, but was it Gijsbrecht's? Opinion is very divided. You can see some of the walls in the Kolk car park.