It’s really strange that it’s square and not round.

It’s not even slightly strange. These are incredibly common all across Europe and the world.

It’s a small cover, almost certainly to service the nearby drainpipe or perhaps for the water or gas connection to a house.

The chance of it falling in the hole is zero unless you’re trying to do it on purpose. Lift and slide, done.

And even if it does fall in, you can simply reach in and pull it out.