Idk how what to do! Our house has the most ridiculous design feature ever...

And I don’t know what to do to make it safe! As you can see in the pic below, we have a hole in our living room floor leading to a bomb shelter. We live in a historic home and we loved the house for this feature alone, but it’s becoming more and more unsafe as our lil guy is rolling! We can’t afford to move and we can’t cover it because hubby has his work space down there. I’m really at a loss of what to do. Literally taking any and all suggestions. Baby gate?? Redo the door? I just don’t know!