With traps blocking wagon access, I figured I'd post my simple remedy to the problem since it seems to be a recurring question or concern for people.
The following design is easily incorporated into any setup, and takes minimal effort to maintain.
The bridges lift towards the Depot, so they cannot be destroyed by building destroyers. The longer hallway can be filled with traps (including a dodge-me trap run), and dogs at the last curve catch sneaky types before they enter the fortress proper. Obviously this is a simple hallway to demonstrate proof of concept; the actual trap hallway you use can curl around, have cave-in traps and whatever else you like. The important point is that your entrance has the Depot just off what would ORDINARLY be the main entrance to your fort.
When not in use, simply close the door closest to the fortress (that would be the Southern one in this screenshot), allowing constant Depot access. Two dogs chained north of the depot will allow thieves to be spotted before they can pilfer supplies left at the Depot, but the raised bridge will force building destroyers to patch the long way around through the trap corridor.
When a merchant caravan arrives, simply swap the bridges; the Northern bridge raises, securing the caravan from any unexpected guests. The Southern lowers, allowing haulers to load and unload the Depot.
Both bridges can be raised to lock the caravan in, either to seal them off during a siege (for safety reasons), or to spring a horrible deathtrap on them (for other reasons or no reason whatsoever).
There you go, a simple and easy way to keep your trap lined entrance, Depot access, and prevent thieves from sneaking past. NOTE: For above-ground forts, all the usual safety concerns about aerial attack still hold true, so don't forget walls and ceilings.