Several forts were built by Paleo-Indians (10,000 to 8000 BC) in today's southern Illinois. These stone forts were built to defend against other tribes or wild animals. They are all located within a 40-mile radius of each other and were evidently made by the same people for the same purposes. These prehistoric stone structures are the only ones of their particular construction in North America.

The forts of the 17th Century were simple fortified log cabins. The next step up in protecting the structures was "Blockhouses." A Blockhouse fort was generally a defense against Indian attacks. The lowest order of this fort class was a single house, built strong and a story-and-a-half or two stories high. The ground level was built with embrasures (holes) to shoot through and fitted with solid puncheon doors, three or four inches thick, with strong bars to prevent entry. The second story projected over the first floor by three to four feet. Outside of the first floor's footprint was used to shoot down, eliminating fire-starters and hiding places. 

There are Four Distinct Types of Fortresses:
  1. Fortified Structure: a secured pre-existing house or a fortified cabin.
  2. Military Outposts: a small group of buildings used for trading and guards against attack on nearby forts. 
  3. Factory Fort: Produces necessary protection item(s) and storage for dry goods, aged meats and fowl. There is room for citizen emergency accommodations (i.e., Fort Dearborn).
  4. Cantonment: A military cantonment has a permanent residential section (i.e., barracks, rowhouses, etc.), serving the fort and other military installations." (i.e., Fort Wilkinson).
Fortifications were first built in North America by Paleo-Indians between 10,000 to 8,000 BC. Upgrades and improvments have never stopped since.

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