A bridge situated on piers, or, as an exception a sort of dike crossing the ditch, permited the access in the fortification. Sometimes, digging the ditch, it was spared a part of land which served as a passage; but, usually, it was prefered an easy bridge, which offered the advantage to narrow the passing and which, in the case of a siege, it was destroyed or it was put inside of the castle.
Before long, it was invented the built in which the superstructure of the bridges it could rise, closing, in this way, the access. This invention, which was named "mobile bridge", perfected fast. The mobile superstructure was manoeuvred through a system of counterpoises, for a less effort to be sufficient to rise it or to let it down.