I've read somewhere in the forum that, in the new version, drawbridges were no longer immune to building destroyers (reports of a dragon who destroyed a raised drawbridge, IIRC). Since the megaproject I'm planning uses them as part of its defense, I decided to make a quick experience.
I used the standard 'dig all the way down to HFS' method. Along the way I placed (from bottom to top): a locked door, a drawbridge raising to the inside of the fortress, and a drawbridge raising to the outside (I seem to recall reading that the direction they raise could make a difference).
The results were the expected, they quickly destroy the door but are completely stopped by either of the bridges. Megabeasts and titans could work differently, I suppose, but that's harder to test since I need to wait for them to attack. Anyone has any experience with megabeasts and drawbridges?
I've also noticed something curious: even though they can't destroy the drawbridges, they still run up to them and pile up in the other side trying to get through. Since the bridges were raised before I breached into HFS, I can only assume that raised bridges, despite acting like walls, don't block pathfinding, at least for building destroyers (much like doors impassable to pets don't actually block their pathfinding). For confirmation, I closed the access to my fortress with a wall and lowered the drawbridges, and they ignored the new wall completely.