What determined which defensive structures were built and where they were located for 17th European century cities?

Question

Using this 1675 map of Maaseik, Belgium, for example, there are hornworks, bastions, and ravelins at specific locations around the outer wall of the city. What determines why certain structures are built at those sites. Is it simply terrain, or are their other factors that determine the layout of the features.

The map of Maastricht, from twenty years earlier, shows the placement of the elements seeming more random.

Toonneel der steden van de Vereenighde Nederlanden : met hare beschrijvingen

Scene of the cities of the United Netherlands: with their description Imprint [Amsterdam] : Joan Blaeu, Year 1651

Answer 1

1. There were treatises on the "scientific" placement of defensive works, notably by the Marquis of Vauban. But he was neither the first nor the last to write this up. They were designed to put flanking fire on any part of the wall from batteries further in. Distances and angles were determined by weapon performances.
2. The geometry then had to be applied to the actual geography. There might be rivers or canals to help the defense, or to complicate it. Hills might have to be secured by outworks. Roads need gates.
3. How much of this "theoretically perfect" defense one could give to a city or fortress would depend on the budget of the state. Each additional layer would prolong the expected siege, but each got larger and required more troops to man the perimeter.
4. Budget problems could be relieved by using parts of existing fortifications. Those might be from a different era, when different weapons dictated different fortifications.

I cannot provide an expert analysis of Maastricht's defenses, but three things seem obvious when one thinks about them:

• In the Maastricht map you link, the first obvious feature is the river Maas, splitting the city into the main city and the defended bridgehead on the eastern bank. The bridgehead defenses are on a smaller scale.
• There is also the smaller Jeker on the south side of the fortifications, and fewer/smaller hornworks in the south where water helps the defense.
• Another thing to look at is the placement of city gates, and how the roads from them cross the defensive works. They tend to go through or between ravelins rather than the larger hornworks.