The current lines were constructed between 1858 and 1871. They included special fortified bridges for road and rail access. A model of the Hilsea Lines featured in the 1862 International Exhibition. Even before their completion the Hilsea Lines had been rendered obsolete by the 1859 Royal Commission and advances in artillery technology; as such they were the last full bastioned trace constructed in the United Kingdom. The development of rifled artillery meant that it was now possible for an enemy to occupy the high ridge of Portsdown Hill north of Portsmouth and shell the naval base from several miles away without having to attack the Hilsea Lines. To counter the new threat a series of "Palmerston Forts" were constructed on Portsdown Hill and the Hilsea Lines were scaled back. In particular a pair of forts that it had been planned to place behind the lines were not constructed.