During most of the fur trade era in the Great Plains, trading post architecture typically followed a ground plan ultimately derived from military fortifications, thus the common use of the word "fort" in trading post names. The strong, defensive nature of trading posts was a by-product of the very nature of the trade. For the (typically) European American builders, the fortified post provided asylum from a rough and dangerous world far from the comforts of home. The fortified architecture also afforded protection for valuable trade goods and provided sanctuary from sometimes hostile Native American trading partners. Native American traders coming to the post may have perceived conflicting messages of promise and threat in the strong defensive character of the fort. The structures were a source of seemingly endless material goods, but the goods could only be acquired from heavily armed and sometimes incomprehensible traders.

Trading posts typically incorporated a structural assemblage encompassed by a square or rectangular palisade. This enclosure was generally constructed of vertical timbers set in a trench and about twelve to eighteen feet in height. Two square bastions or blockhouses were often built on opposing corners of the palisade. More rarely, these had a circular floor plan. The bastions generally had a pitched roof and loopholes from which small cannon and shoulder arms could be fired. To impede hostile intruders from climbing over the top, palisade pickets were occasionally sharpened or surmounted with chevaux-defrise (crossed, pointed sticks). A gallery (or building roofs in smaller posts) was usually built about four to five feet below the top of the pickets to allow sentries to patrol and fire from the palisade perimeter.

Inside the palisade, buildings were raised around a commons area or open courtyard outfitted with cannon and flagpole. In smaller forts, buildings were constructed directly against the palisade's interior walls. Larger posts usually had structures set out from the palisade with the space between the palisade and structures frequently used for storage or as stables. A house, or "mansion," for the bourgeois or fort superintendent, generally the most imposing structure in the complex, was usually placed opposite the main entrance to the post and would have been the first structure seen by those entering. It often displayed painted wooden siding. Aside from serving as a home for the superintendent, clerks, and guests, the mansion would house the post's business office and dining hall. A separate kitchen was placed behind or near this building. The remaining three sides of the courtyard incorporated ranges of lesser structures. These were usually constructed more crudely without siding and left unpainted. Earth or sod was most commonly the roofing material of choice for these buildings. Structures in the ranges served the trading company as icehouse, powder magazine, employee residences, fur storage, trade and dry goods storage, blacksmith shop, and trade store.

With rare exceptions, trading posts were constructed according to vernacular building traditions common to the region in which they were raised. Aside from typical notchedlog structures, vernacular French Métis construction methods were commonly employed in the Canadian Plains. Poteaux en terre (posts in earth) structures utilized a wall or building frame of vertical whole or split timbers placed into trenches in the ground. A more challenging construction method was poteaux sur sole (posts on sill), which placed vertical posts on wood or stone sills to prevent wood rot and thus provide greater structural endurance. Spaces between vertical framing were packed with mud and grass (bouzillées) or stone and plaster mortar (pierrottées).

Another common construction method seen on the Canadian Plains was Red River frame, or pièce sur pièce (timber on timber). Like poteaux sur sole, this French Métis construction method used vertical logs raised on a wooden sill. In this instance, however, the vertical members were grooved. Spaces between vertical members were filled with horizontal logs whose tongues or tenons were inserted into the vertical grooves to make a wall. Interior walls were commonly plastered and whitewashed.

Occasionally, adobe was used in the construction of trading posts on the Northern Plains (e.g., Fort William, North Dakota). This building material was more commonly used on the Southern and Central Plains, reflecting the greater Hispanic influence in these regions. Examples of adobe trading posts include Fort John (later Laramie) in Wyoming, Fort John in Nebraska, and Bent's Fort in Colorado. Adobe provided an excellent substitute for wood in the relatively dry climate of the High Plains, an area with few trees. Aside from its excellent insulating qualities, adobe construction allowed more free-form structures. Bent's Fort, for example, had circular bastions and generally displayed an abundance of curved structural elements. As such, it had a less forbidding look to it than Fort Union in North Dakota, for example.

Trading posts in the Great Plains were generally of three types, their structural complexity reflecting their position in the fur trade business hierarchy. At the pinnacle of the hierarchy was the major post. These fortified complexes (such as Fort Union, North Dakota, and Fort Garry, Manitoba) functioned as the regional headquarters of a company or trading outfit. They supplied and directed the trade of a number of subsidiary trading posts and wintering posts. Like posts subsidiary to them, however, major posts also served as a center for trade over an extensive area with one or more Native American bands. Major posts were usually quite large, imposing in appearance, and designed to last for a relatively long period of time.

Subsidiary posts such as Fort McKenzie, Montana, and Fort Qu'Appelle, Saskatchewan, were under the administration of a major post. Subsidiary posts were located in the general wintering area of an individual tribal group or band and operated year-round. Although usually smaller in size than the major posts, they were fortified similarly. In keeping with their secondary status, subsidiary posts were constructed more simply and usually built to last for a much shorter duration.

Wintering houses were the smallest and most temporary of trading posts. Administratively situated under one of the other two kinds of posts, they tended to be unfortified and often consisted of little more than a rude log cabin or even a skin lodge. Wintering houses generally formed the "front line" of the fur and bison robe trade, and as such they were usually placed in or near the village of a nomadic group. Since the nomads rarely wintered in exactly the same place from year to year, these establishments typically served the trade for a single trading season. Small-scale independent trading companies tended to resort to this type of post as a necessity, their financial resources being too meager to allow construction of larger palisaded posts.

Fortified trading posts in the Central and Southern Plains were largely replaced by small entrepreneurial operations during the 1840s. The large trading posts of the previous era were similarly reduced in scale to small log or adobe structures. On the Northern and Canadian Plains, however, large enterprises continued to operate fortified trading posts for several decades past the demise of their southern cousins. By 1870, however, even that region's elaborate trading posts were being replaced by small posts comprised of simple, unfortified log structures similar to those that had been built on the Southern Plains in previous decades.

See also INDUSTRY: Fur Trade.