We found the same type well at our ranch, its about 50 ft deep. In fact Texas has a website that lists nearly every water well in the state, shows this one was dug in the 1890s. You can see grooves in the top stones where people repeatedly pulled buckets up with ropes. No way in the world I'd cover that up, in addition to it being cool, pretty sure the poor guy who dug it would crawl out of his grave and shoot me.
I welded a few pieces of steel pipe together in a grid and laid them across the top, then placed a sheet of heavy guage welded wire (we call it goat or hog panel here, it has about 2"x 4" openings). Then I built a temporary wood frame around the edge and poured an 8" x 8" concrete curb around the perimiter to hold the steel in place. Its sturdy enough for any grown man to stand on, and impossible that anyone could accidentally fall in, but you can still look into the well and appreciate the history of it. Even wrote the date the original well was dug in the concrete and let the kids write their names and the current year.