The Cahokia Courthouse was built as a residence around 1740, when present-day Illinois was a colony of France. In 1793 the structure was purchased by the Common Pleas Court of the United States Northwest Territory and subsequently became a center of territorial political and legal activity. The building is historically significant as the oldest courthouse in Illinois and the only one remaining from the state’s territorial period (1787-1818). It is architecturally significant as an example of the French Colonial vertical log poteaux-sur-solle (“post-on-sill”) construction technique.