system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did old forts actually "protect" a strategic area? Couldn't the enemy just go around them or stay out of range?

I've visited quite a few colonial era and revolution era forts in my life.  They're always surprisingly small and would have only housed a small group of men.  The largest one I've seen would have housed a couple hundred.  I was told that some blockhouses close to where I live were used to protect a small settlement from native american raids.  How can small little forts or blockhouses protect from raids or stop armies from passing through?  Surely the indians could have gone around this big house.  How could an army come up to a fort and not just go around it if there's only 100 men inside?  

**tl;dr** - I understand the purpose of a fort and it's location, but I don't understand how it does what it does.
gpt
Yes you are right you can try and avoid it. 

The issue with forts and castles is that it's a hideout for men. You could ignore it but you then have a group of highly armed and trained men you can nip out, attack your supply chains and retreat back into the safety of the fort. 

Forts are often built in higher places giving a larger radius of fire meaning you would have to go a very long way around to avoid them. This would often be through terrain so would slow troops down and maybe impossible for carts to traverse. Rivers with bridges or fords were a commonplace to build forts near as defending troops can snipe, enemies, trying to use the crossing. 

If you went past the fort you'd have real problems supplying or even having your frontline troops retreating. Runners were needed for communication so messages could be hindered or intercepted.

Finally, it's hard to prove to the populace that you're their new ruler if they're old ruler is still down the road looting your supply convoys.