Thanks Djekspek. I used the description and a iconic map from Wikipedia as a template for this map and there weren't any bridges. The only connection between the different districts of the inner town is by the channel. I think this can be described as some form of protection. According to Platon the Atlantian capital fleet alone numbered 1200 ships with 240.000 men of crew!
But the old Greeks tended to exaggerate nearly everything