17th-century Amsterdam needed protection. In fact it needed more than that. In order to maintain its very existence, it needed a clear demarcation. A physical barrier between city and country side. An obvious distinction between in or out. The ring of fortifications that surrounded 17th-century Amsterdam provided the city with such a distinction.
Construction of the fortifications started in the late 16th-century. Before that time, Amsterdam was surrounded by a wall. Due to changing military strategies, a ring of fortifications with several strongholds became the new standard of defense in early modern Europe. This way, cities were better and easier to protect. The great extensions of the city in the 17th-century meant that the fortifications needed to be built anew, on a very large scale.
The expansion of Amsterdam in the 17th-century, resulting in the famous (UNESCO World Heritage) canal-belt, was carried out in two stages. The first extension on the west side of the city was constructed roughly in the years 1610-1616, the second one between 1655-1665. Before they could start digging out the canals and selling the new plots of land, the defense structure of the city had to be in order. This meant that the fortifications had to be built first. And though carried out in two stages, the realization of the defense structure alone was a major challenge. In fact, because of the enormous costs of building the fortifications, the city decided that the expansion would not be carried out in one go.
The completed ring of the defense structure was about 8 kilometers long. There were 26 strongholds pointing outwards, and people could enter the city through one of the 8 gateways (only one still remains). The fortified ring was about 5 meters high and coated with stone. On top of the strongholds there were windmills for various uses (one of them remains today). The ring of fortifications of the city was surrounded by a wide canal, span by drawbridges to reach the gateways.
Today the old ring of fortifications no longer exists, although you can still see some of it if you know where to look. The canal that once surrounded the whole structure is still there. It still curves around in places where the strongholds used to be. In fact, one of the strongholds is still clearly visible as a sharp point sticking out in the canal. Just keep in mind that when you cross the last canal that girds the old city, you are crossing the dividing line between in or out. Or nowadays, between old and new.