After the ditch is crossed, it arrived at the gate of the principal enclosure. The gate opened to the left, because, in this way, the besieging was forced to expose to the fortifications his right flank, which wasn't covered by the big shields named "pavois" which used in attack.
The gate of a castle is placed always near, in a compact massive of masonry formed of two towers bound between them from a part of the building more or less important. It presents a passage narrow enough, which can close at the two extremities and, sometimes, even in the middle.
The mobile bridge, finally risen, had somehow the function of a large shield against the enemy; but this was able to succed in throwing down, tearing the chains which held it suspended, with harping irons, with the strength of the arms or with machines. So, it was necessary to opposed such an obstacle. This was the "hersa", a heavy iron lattice work or an independent system of poles; the second made of closing it was named saracin, an expresion which seems to indicate that this invention had an oriental origin. This machine rose or let down, sliding in holes made in the passing walls.
The "hersa" was risen with a device, and when a danger approached it was let to fall down. From this moment, the passage was closed and it had to be broken the "hersa" to penetrate further, because it was impossible to be risen from the outside.