Why were forts and whatnot so important to stall enemy advances? Its not like they built walls spanning the entire width of france! ...did they?
What would Spain need to do something like this? Would the requirements be unreasonable? If largely impossible, please explain what would be so obstructive.
This question is not easy to find answered anywhere else, so thank you very much.
The problem of circumventing forts back then was that it allowed those troops to have a chance of flanking you if you were out on the march. If you look at Napoleons campaign in Italy in 1796, there was a rather significant fort called Mantua which Napoleon laid siege to more than once. This was because that this fort was vital to the Austrians line of communications and held a significant garrison that could potentially threaten the French if left unmolested.
Yes, the spanish could just march its army up to paris and circumvent every fort they cane across but there were literally dozens of forts that outlined Frances southern border with Spain and in every one of these forts had a significant garrison that, if pooled together, could potentially outflank an invading army. Which is why forts, depending on the circumstances, were always sieged and taken by storm.