A retracting bridge in its off state would cover a hole, and would be invulnerable from below. They are not invulnerable on the same z-level / top. I assume, but am totally unsure, that they are invulnerable when "on" / retracted.
If you have a few z-levels to work with, I would reroute your entrance. Dig a new tunnel, then make new stairs to the surface. Once there are two routes to the surface, wall up the old path. If you want to use a retracting bridge, then once you have a tunnel, dig down a level, then dig an alternate up staircase to the surface. You can cover that down staircase with a bridge. Or you can just build a raising draw bridge in the tunnel.
Personally, I prefer a multi-tiered defense. I have a long winding corridor for caravans, the corridor has a raising bridge at the front. There is a shortcut with some traps that runs to the barracks. The barracks leads to the trade depot, as does the long corridor. From the depot, I have 2 doors that lead to another raising draw bridge. Behind the last draw bridge I have 2 more doors, a gap, and then my main stairwell. If I know I can't handle a threat, I seal the main corridor. If my military is defeated, or my bridge doesn't close in time, I lock all doors, and close the final draw bridge. That buys the lever-puller time. If both bridges are compromised, the final set of doors buys my masons time to build a wall.