In addition to this sea-wall which skirted the coast, the mouths of the three harbours were contracted by moles of solid masonry that ran out to meet each other on either side, leaving only a narrow entrance between their extremities. The long moles which thus barred the mouth of the great harbour still exist, though the southern of the two has been washed away by the waves to a depth of some thirteen feet under the surface of the water. They now support the red and green lights which at night mark the entrance to the harbour. The haven of Zea is naturally stronger than the great harbour, and therefore needed less elaborate fortifications. It consists of a circular basin lying about two hundred yards inland from the sea, and is approached by a channel a hundred yards wide. Walls ran along this channel on either side, so that an enemy’s ships endeavouring to enter the harbour would have had to run the gauntlet of a cross fire. At its inner end the channel was flanked on either side by a tower of solid masonry built out into the water, but connected with the fortification walls. The third harbour, Munychia, the smallest of the three, is farthest removed from the business and bustle of the modern port town, and hence has, in some respects, best preserved the relics of antiquity. Originally it was a mere open bay, and therefore needed vast constructions of masonry to convert it into a war harbour. The moles built for this purpose are described by Lieutenant von Alten, who examined them with attention, as the most magnificent specimens of ancient Greek fortification which have survived. In some places on the outer edges of the moles the colossal blocks of which they are composed have been piled up in wild confusion by the heavy surf, and project like islets above the surface of the water. Each mole ended in a tower; and the narrow entrance to the harbour was between the towers. The tiny basin is commanded by the hill of Munychia which rises steeply from the shore. In time of danger each of the harbour mouths could be closed with a chain stretched between the two towers that flanked the entrance. The chain seems to have been coated with tar to prevent it from rusting in the water.