A Greater than Napoleon by Sir Basil Henry Liddell Hart

On the seventh day from the start of the march Scipio arrived before the city and encamped, the fleet arriving simultaneously in the harbour, thus cutting off communication on all sides. This harbour formed a circular bottle, its mouth almost corked by an island, while Cartagena itself was like a candle stuck in the bottom of the bottle, the city standing on a narrow rocky spit of land protruding from the mainland. This small peninsula bore a distinct resemblance to Gibraltar, and the isthmus joining it to the mainland was only some four hundred yards across. The city was guarded on two sides by the sea, and on the west by a lagoon. Here was a hard nut to crack, seemingly impregnable to any action save a blockade, and this, time prevented.

Scipio's first step was to ensure his tactical security by defending the outer side of his camp with a palisade and double trench stretching from sea to sea. On the inner side, facing the isthmus, he erected no defences, partly because the nature of the ground gave protection, and partly in order not to hinder the free movement of his assaulting troops. The Carthaginian commander, Mago, to oppose him armed two thousand of the sturdiest citizens, and posted them by the landward gate for a sortie. The rest he distributed to defend the walls to the best of their power, while of his own regulars he disposed five hundred in the citadel on the top of the peninsula, and five hundred on the eastern hill.