Fort Carnot is a rough, atmospheric remnant of French colonial rule above Houei Xai. The old walls and structures are weathered rather than polished, but that is part of the point: this is not a curated heritage show, just a hard-edged leftover on a hill above the Mekong.
The views are the practical reward, with the river, town, and Thai bank helping you place Houei Xai as a border settlement. Go with realistic expectations and decent footwear. The fort is most interesting if you like places where history has not been tidied up too much.