Place a 2x2 floor piece on each side of the gap. Connect 2m beams from one floor piece to the other across the gap (you might need more than one beam if the gap is bigger than 2m), leaving a space between them to allow for cart wheels. If one side is higher than the other, use wooden steps to make a ramp. You can then pull a cart across no problem but enemies will not cross. I had a skeleton attack and they all stood on the other side and wouldn't cross.