I can think of several weaknesses with the Roman Empire . For the size of the Roman Empire the army is too small and overpaid due to imperial power resting with the army. Romans have castles but they're mostly used for garrison's they have no castles or notifications protecting the interior of the Roman Empire. So should the legion fail then Rome will be open to attack. Having a powerful German state could revitalise regenerate the Roman Empire or lead it to destruction. Also I forgot to mention the Roman empires succession problem being a monarchy with republican characteristics. Also add as a final note Constantine the Great is classed as your Europe's first medieval monarch.