The French & Indian Wars were pretty famous for forts & sieges. If that era suits your story, it's pretty fascinating history. --
French & Indian Wars Forts
Fort William Henry's probably most well-known for showing up in James Fenimore Cooper's "Last of the Mohicans" but there were heaps of them. Basically the frontier at the time was a wobbly line that sort of bulged outward from the forts, and they were constructed strategically on rivers, near valley pinch points, on lakes, etc.
The 19th Century forts and battles tended to be further west, ala Kansas, Nebraska, etc -- but by the Civil War firearm and artillery had advanced to the point where constructing new forts wasn't really worth it. They're good tactically against a faster, weaker force, but once you start throwing advanced artillery into the mix they lose a lot of their effectiveness.
Hope your story goes well! You've got lots of material here, I think.