The Einzberns from the beginning were a factory of homunculi. Per the booklet released alongside the DVD for the Unlimited Blade Works anime:
• The origins of Einzbern
A factory originally created by the students of the magician who
realized the third magic. Began in the year 1 AD. They attempted to
reproduce their master’s miracle, but couldn’t do it themselves, so as
an alternate plan they tried to recreate a specimen identical to their
master and have it reproduce the magic.
After nearly 900 years of effort, Justeaze—the homunculus who would
come to be called the Winter Saint—was forged. She was a model that
strayed from what the magi had intended, created completely by
accident, but her abilities were equal to or above those of their
master. The magi should have been elated about Justeaze, but they
couldn’t be happy. After all, she was a mutation that was born
regardless of their own techniques or skills. Even if that specimen
was to reproduce the third magic, that conclusion would be harder for
them to bear than 900 years of failure.
The magi deperately attempted to create a homunculus that excelled
Justeaze by their own techniques. The artificial intelligence created
to act as the castle’s central management—the golem Jubstacheit—was
the pinnacle of their craft, and became the father of all homunculi
created at Einzbern.
Justeaze succeeded at proving the third magic. However, it was not
cost-effective. Justeaze’s use of the third magic was like careful
knitting. It would take several years to save just one person, making
the salvation of the entire human race virtually unattainable. In
addition, while Justeaze herself did not age, her body was so frail it
would take little to kill her, so she could not leave the castle.
Since her intelligence and mentality were also unaging, she did not
develop at all. From an outside perspective, it was as if she was
repeating a single day for all eternity. If she were to leave the
castle she would be freed from this “single day,” but the act of
leaving the castle signified an easy death for Justeaze.
In the end, they accepted that humans could create something that
surpassed humans, but not something that could save humans. … Or
perhaps they would not have failed if a miracle like Justeaze had not
The magi despaired the limits of their abilities. Some abandoned the
castle, while others took their own lives. The homunculi left at
Einzbern were abandoned by their creators, but in their purity, they
continued to operate the factory for the sake of the creators’
ideology—the salvation of humankind, the reproduction of a miracle.
From then on, all homunculi created at Einzbern were based off of
Justeaze. Jubstacheit created a humanoid terminal unit, and used it
as the manager of Einzbern. Though by the end he operated even Acht
(the eighth humanoid terminal), he did not possess a personality. He
only included the necessary “humanity” to operate the castle and
reproduce the third magic into each of his humanoid terminals, and
made them act like humans. Jubstacheit is essentially an automaton
that cannot progress, but continues operating in perpetuity. His way
of existence is just like an old clock ticking away, being forgotten
by people before running down.