I will keep this short. In our timeline, the Romans built a new capital, Constantinople, on the Bosphorus. Strategically located on the crossroads between Europe and Asia and easily defendable due to the Golden Horn, the site was a great place for a city. However, are there any other sites in Europe that were easily defendable and could have a huge classical city built on them? It'd be ideal for this new capital to be slightly more West than Constantinople, but probably not as far West as Rome itself. Any ideas where else they could have built a new capital?