There were four main "systems" of forts throughout US history.

3 systems, and a 4th special type system that dealt more with ordnance being employed from hardened positions, i.e. not the shape of a typical truncated hexagonal fort with traditional walls, bastions, etc, but instead reinforced singular or grouped gun positions. Fort De Soto is a good example.

1st System was what we see in the frontier area of like say Florida, etc. Dug outs with wood from trees reinforcing the positions.

2nd System was what places like the 1st system showed, but had incorporated stone/concrete, and had employed cannons and similar ordnance at strategic points of the forts.

3rd System were the culmination of the fort systems of America, these being most all part of an intricate coastal defense system that lasted until humanity had begun to mass produce intercontinental aircraft, thus making these forts obsolete. A good group of Florida forts to study are Castillo De San Marcos, Fort Clinch, Fort Barrancas, Fort Morgan, and for a late 3rd System/Early 4th System example, Fort De Soto in my home of Pinellas County.

The 4th System had come about during a rapidly developing arms era of the early 20th Century just as aircraft had come into play. It had new ideas put into play, such as the use of extremely heavy "disappearing" guns in lieu of casemate, or wall top mounted howitzers or coastal guns. Anti Aircraft Artillery had become a factor as air power began to trump the paramount use large fleets of dreadnought ships, and other coast threatening vessels.

By the end of WWII, we can see that all the used forts had become obsolescent, and slowly became mere reminders of a way war was once waged.