It is formed of massive blocks of sandstone, which are placed transversely, so that their length forms the thickness of the pier, and their ends the wall on either side.

On both sides of the wall are quays of concrete.[439] The line of the ancient enceinte may still be traced around the three outer sides of the island.

It is a gigantic work, composed of stones from fifteen to eighteen feet long, placed transversely, like those of the centre pier, and in two places still rising to the height of five or six courses (from thirty to forty feet).[440] The blocks are laid side by side without mortar; they are roughly squared, and arranged generally in regular courses; but sometimes two courses for a while take the place of one.[441] There is a want of care in the arrangement of the blocks, joints in one course being occasionally directly over joints in the course below it.

The stones are without any bevel or ornamentation of any kind.

They have been quarried in the island itself, and the beds of rock from which they were taken may be seen at no great distance.