Buddhist temples Sanchi and Dhamek in India. That is short of counting various tombs (like the Giza pyramids and Scotland burial sites) as your condition of continuing use.

hi! there have been a few similar posts; see here for a bunch of previous responses

Which building is the oldest in the world, but not a ruin and is still used?

What's the oldest building in the world that has been in continual use/occupation?

What is the oldest known human structure?

What kinds of ancient structures are still in use today?

The tower of Hercules is a lighthouse on the northwest cost lot Spain in. La coruna. Been. In use since Roman times ,although not huge continuously.