Realm of History sports a rather good article, in list format, on the early mediaeval Eastern Roman (also known as Byzantine) army. Recommended little read.

Note that the ancient Roman legacy of organized military professionalism (including siege engineering) was carried on in the realm of Constantinople, albeit much reformed over the centuries as the state and army was forced to adapt to new realities. While not on par with the legions in their golden heyday of conquest, the Byzantine army for much of the mediaeval Roman empire's history was nevertheless a formidable force which, along with heavily fortified cities and a strong navy, managed to stave off annihilation of the empire despite the wave upon wave of enemies that attacked her lands from all sides through many centuries of devastating warfare, and even managed to push back and reconquer lost lands every now and then.