I have a 1916 house with a basement with a concrete floor. There is a round 12" diameter hole (sump) in the basement floor about 18" deep. The basement sink and washing machine, kitchen sink and basement shower drain into it and it drains into the main sewer line. There is a rusted iron top on it. Do I need something to prevent the sewer backing up into this thing and flooding the basement? What is it?

Just regular maintenance on the pump.

If you have the funds to do so, I'd suggest a back-up pump and an alarm system on the main pump.