By far the greater number of these posts lay along the Missouri River and most of their names have long been buried in oblivion. Many are permanently lost, while others can not be cleared of uncertainty as to their true location and ownership. Many of the names are perpetuated in towns and villages which have grown up on or near the old sites. Others, that should have survived on account of their great importance, can no longer be found today.

These establishments were generally designated as "Forts." Their primary purpose was trade, but in a land of savage and treacherous inhabitants they served the purpose of protection as well. Their construction was therefore adapted to both ends. The ground plan of the typical trading post was always a rectangle, sometimes square, but generally a little longer in one direction than the other. The sides varied in length from 100-400 feet depending upon the magnitude of the trade which the post must accommodate. In order to ensure the necessary protection the fort was enclosed with strong walls of wood or adobe. There were a few posts built of adobe, but these were the exception. The typical fort was protected by wooden palisades or pickets varying from 12-18 feet high and from 4-8 inches thick. In some instances the pickets were squared and set in juxtaposition; in others they were round pieces formed by sawing logs in halves. They were set from 2-3 feet in the ground and the earth was generally banked up to a small height against them. In some forts there were musketry loopholes along the top of this embankment. For the purposes of guard duty and also for active defense, a plank walk was bracketed to the inside of the pickets about four feet below the top so that sentinels could walk there and observe the ground outside. In case of attack the defenders could mount this walk and fire over the palisades or through the loopholes provided for the purpose.

The main reliance for defense consisted of two bastions, or blockhouses, as they were commonly called, placed at diagonally opposite corners of the fort. They were square in plan, 15-18 feet on a side, with two stories, and were generally covered with a roof. The lower floor was a few feet above the level of the ground and was loopholed for the small cannon which all the more important posts possessed. Above the artillery floor was another for the musketry defense with about three loopholes on each exposed face. The blockhouse stood entirely outside the main enclosure, its inner corner joining the corner of the fort so that it flanked two sides; that is, the defenders in each bastion could fire along the outer face of two sides of the fort and thus prevent any attempt to scale or demolish the walls.

A "fort" thus constructed was really very strong and was practically impregnable to an enemy without artillery. A host of Indians armed with bows and arrows or with the indifferent firearms of those days could make no impression upon it, and the garrison could look with indifference upon any attack, however formidable, so long as they used reasonable precaution and were supplied with provisions and ammunition. There is no record of a successful siege of a stockaded fort in the entire history of the fur trade west of the Mississippi River.

The necessary prerequisite of defense having been satisfied, the other arrangements of the fort related to the purposes of trade. The entrance was through a strong and heavy door provided with a wicket through which the doorkeeper could examine a person applying for admittance. In the more elaborate posts there was a double door, with a room and a trading counter between them. The Indians were admitted only to this space for purposes of trade. In the single-door posts trading was sometimes conducted through the wicket when there was suspicion of danger.