On the landward side the peninsula was defended by a wall, which started from the harbour of Munychia, ascended the hill, and after following the edge of the plateau for some distance gradually descended westward to the shallow northern bight of the great harbour, across which it appears to have been carried on a mole or dam. This landward wall, to judge from its existing remains, seems to have been a masterpiece of military engineering, every opportunity offered by the nature of the ground for strengthening the fortifications having been unerringly seized upon and turned to account. The naturally weakest spot in the whole circuit was where the wall crossed the flat between the hill of Munychia and the great harbour. Here accordingly we find the wall especially strong; it is twenty-six feet thick, and is constructed of solid masonry in large squared blocks without any core of rubble. Naturally the gates were placed in this landward wall and opened northward. Remains of four of them can be distinguished. The principal gate, flanked by two square towers on oval bases, stood in the flat ground between the north-east end of the great harbour and the heights of Munychia. Through it doubtless ran the highway to Athens; and here at a little side portal for foot-passengers probably stood the image of Hermes, which the nine archons dedicated when they set about fortifying Piraeus. A little to the east of this principal gate and on slightly higher ground is another gate, through which the road to Athens went between the two Long Walls. The gate is double, that is, it is composed of a court nearly square with a gate at each end. The reason of this construction, which is common in Greek fortifications, was that, if an enemy should force the outer gate, he would still have a second gate in front of him, and would in the meantime find himself pent in a narrow court, as in a trap, from the walls of which he would be assailed on all sides by the missiles of the defenders.