THe old city area is surrounded by a moat with gates in the middle of each long stretch. It is rectangular, the long sides measuring about 1.8 kms, the short sides, about 1.3. The walls were/are brick. As with any mediaeval city, to create this sort of structure, must have been a massive effort. Fairly obviously, the wall pieces which remain are only tiny compared to the original, and even at that, some of the sections are reconstructions. Still, it gives the idea.