Not just in Assyria, but in royal courts across the Middle East it was common practice for eunuchs to live in the palace. Eunuchs were called sa resi in Assyrian, which translates as
those of the head — this referred to the fact that they did not have a beard (and suggesting some palace staff were intact, bearded men) and was an ancient title for a personal attendant. They were the archetypal servants of the king.
However, these eunuchs were clearly not just personal attendants — from the reign of Ashurnasirpal onwards, they were sent out to the reaches of the empire to exert and represent royal power. The decision to empower eunuchs means that in Assyrian history we see very, very powerful eunuchs — the likes of Dayan-Ashur, Shamshi-ilu, and Nagal-eresh. These men were endowed with tremendous power and had deep determination at times when the king was either too old or too young to adequately fulfill his role. We know of eunuchs who served as military leaders, governors, and crucial empire-builders on behalf of the crown.
Their primary role was to serve the king with no attachments nor obligations outside the royal court, and without being able to cultivate their own dynasties nor interfere in the royal dynasty. They did not — nor did they need to — connect themselves to anyone else but the king. This was of paramount importance for the royal family. The Assyrian king's legitimate heirs were any children he begot with his wife or any of his consorts, which was totally different from regular Assyrian households where legitimate heirs had to be offspring of the one legal husband and the one legal wife. Allowing many women to provide an heir made preserving the dynasty somewhat easier, but it made it much more difficult to ensure the paternity of the king. Furthermore, only men could serve as officials. Having eunuchs live in the palace meant there was a highly-skilled professional class to contribute highly valuable service to the palace bureaucracy, but without interfering in the king's dynasty. Eunuchs could not disrupt the bloodline — they were sterile, after all — and they could never take the throne, as the Assyrian king was required to be a perfect and intact specimen of a man. Eunuchs were physically and socially engineered by the royal court for the sole purpose of serving in the exclusively male staff of the royal bureaucracy.
When Assyrians described the initiation of a eunuch, their choice of words translates literally as to enter the palace. Firstly, this suggests that eunuchs were born outside of palace-dwelling nobility and that a eunuch's destination was the royal court. For non-noble families, this was one of the few — or only — ways to forge a surefire connection to the palace. For this reason, social mobility is considered to have been the most likely and primary reason for a family to send a son to become a eunuch. However, we seldom know any information about royal eunuchs — their pasts, their origins, or anything — before their lives as part of the royal court. For this reason, it is hard to reconstruct the typical journey that led to this destination.
Secondly, the phrase to enter the palace emphasizes the aspect of entrance in becoming a eunuch — as opposed to the physical aspect. When a eunuch entered the palace it must have happened very young, as the testicles were crushed before the onset of puberty. Eunuchs were thus in the palace ranks for all of their adult lives and for much of their childhood as well. Being brought up to serve within the palace was the most defining feature of a eunuch's young life. The eunuch was raised, educated, nurtured, and had all his personal attachments within the palace — and by extension, only with other groups and officials appointed to serve the interests of the king and the Assyrian state interests. They must have been a sort of extended part of the royal household. The eunuch was totally divorced from his origins. He was only ever known by his own name: instead of as X, son of Y as was the custom otherwise — and even in death there was no return, as the king was obligated to ensure the proper execution of a eunuch's funerary arrangements (a responsibility usually falling to blood relatives). Also, the king was responsible for ensuring the royal eunuchs' graves were protected in all perpetuity, reflecting the degree of two-way loyalty between eunuchs and the kings.
There was, of course, a physical aspect to becoming eunuch: the crushing of the testicles before the onset of puberty. This ensured a beardless and sterile pool of male staff to fill the ranks of palace bureaucrats, who were completely male. It is unclear if this happened before at the same time as entering the palace, or perhaps it happened a short time after. Perhaps some boys sent to enter the palace were sent back to their families, while only the most promising boys would graduate the selection process to be inducted as eunuchs for the rest of their lives.
Eunuchs were socially distinct — and clearly recognizable. They are the only Assyrian males to be shown beardless. Having never entered puberty, they were smooth-faced and reliefs reflect this unique, distinguishing feature of their personage. They were understood to be male — that was not under question, and he was not socially inferior in any way — but being beardless ensured meant that they would never really blend in with other men. When their roles across the Assyrian Empire increased and they were away from the palace for extended periods, their beardlessness — a pronounced characteristic in how they were represented in artwork — meant they were immediately recognizable as part of the king's close staff of royal eunuchs. Living outside the palace, other Assyrians had a visual indicator for recognizing these men who were linked so closely to the royal court and who represented the interests of the king and nobody else. (The only other beardless men in Assyrian society looked altogether different because they would shave off all their hair from head to toe for religious purity; eunuchs never had a beard to shave but looked otherwise normal.)
Rise of eunuchs in the 9th century
In the 9th century, King Ashurnasirpal decides to send the eunuchs out of the palace to administrate on his behalf across the realm. Because they were likely chosen by merit — not family ties to various noble elites — and had excellent training, they must have had a tremendously positive, stabilizing effect on Neo-Assyrian governance as provinces were formally incorporated. Another benefit for the Assyrian king was that of course a eunuch could never make a dynasty, and when he died, retired, or transferred to another position the successor could only be appointed by the king. Power, ultimately, was only reserved by the king even while delegation had to reach new depths across the expanding empire.