Interesting parallel with Byzantium and the eunuchs, ISTR Narses was both a eunuch and commander in the field at times, though the parallel that draws me is the Ottoman Court and the Jannisaries (there are earlier slave soldier parallels in Islam with the Ghulams IIRC, but I don't know too much about their background). OTOH it seems clear AL was drawing on Roman Auxiliaries for the Ancilliaries.