Having found some new information - first of all remembering that Simon James' book "The Excavations at Dura-Europos 1928 to 1937 - Final Report: v. 7: The Arms and Armour and Other Military Equipment" has some information on the siege, and on the defences, and also having come across some useful photos on the world wide web (thanks Richard!), I decided I'd have to redo the battlements. So here we are - I hope this is the final version. The tower top will also change, and will end up looking much more 'Greek' - with the crenelations looking rather like windows (though the tower won't have a roof) - i.e. the crenelations gain a 'lid'.