This is the south wall and main gate of Fort De Chartre, center of French government in the central Louisiana territory till 1763. https://fortdechartres.net/
Very historic place and you can visit anytime.
It had been bookmarked on our map for several years before we finally visited this spring on the way to a RFKD in MO. I think the powder house is the oldest building in Illinois. An interesting stop, and it pairs well with visits to Fort Kaskaskia and St Genevieve.
Yes the powder house has claim to being the oldest building in Illinois. The fort itself gives a good feel for colonial France. Some of the reinactors there have consented to give a volley fire demo at one of our event nearby. Should be memorable!