Roman era fortifications. The city is remarkably well preserved. It's a national heritage site. People can 'adopt ' wall per meter so on the inside of the ramparts there are amall plaques everywhere to signify who is paying for the upkeep. In Belgium we also have plenty of roman era walls and buildings, but not as pristine as this.
Most buildings inside city walls are from the 1600s or even earlier. As i said our hotel originated in 1603 though the current building itself is about 200 years newer.