I would most certainly not be able to handle my open door policy in a terrifying embark. I lost (primarily)to 3 elite bow goblins and their accompanying ranged units. If I was almost killed by that, I would currently stand no chance. My fort somehow survived even though the goblins were pouring into my fort, I must have inflicted just enough damage to cause them to flee. I was just watching them flood in and knew it was the end, until suddenly it wasn't.
I'm not sure what you could do with pieces that keep getting up to attack you non stop. Maybe alternate courtyard entrances where after one set of battles you clean up with magma or compactors, in the meantime the other courtyard opens up and is the new front door. Although until something elaborate like that is setup, you'd have to shut yourself in for a good while.
It isn't that difficult to set up. Lever to bridge. Same lever to a gear and that gear to the other bridge. Throw lever to open both bridges. Connect a temporary (second) lever to the gear, then throw the lever to toggle it which should close the one bridge. Next, deconstruct the second lever. The gear should then keep the bridges out of sync, allowing you to throw the lever to open one and close the other, at any time.
I believe the only hazard here is if something heavy were to stand on a bridge, keeping it from closing, at which point the bridges might be forcibly synced again. !!science!! (or at least XXscienceXX) would be needed to verify that danger, of course.