The Wizarding world's greatest trait wasn't the power of its magic. It was that its people were all masters hidden in plain sight.
Behind the back courtyard of a tiny, shabby tavern, a crumbling wall in some forgotten corner, or even a building's support pillar that everyone saw but no one noticed, there might be an entire gathering place for Wizards.
Hogwarts, meanwhile, stood in an uninhabited region beneath multiple layers of magic that concealed it from Muggles. A Hogwarts Express railway invisible to ordinary people connected it to King's Cross Station.
The railway had not been built by Wizards. They had hired ordinary people to do the work, then cast spatial magic over the completed line, folding the railway into a layer between the spaces of the real world.
They had also cast Memory Charms on every worker involved in its construction.
Afterward, the railway workers had completely forgotten building the line. They only knew they had come into an unexpected windfall.
There was no need to do that for the base in Shrek City, but Lu Yi could build the base here along the same lines as Hogwarts and its railway.
Since arriving in Douluo, magic had acquired a mana cost. Casting spatial magic on that scale entirely by himself would take him a great deal of time.
But Lu Yi could use the Fidelius Charm to conceal the base completely from outsiders, achieving an even more thorough hiding effect than spatial magic.
The Fidelius Charm could hide a secret completely. That secret could be an event or a location.
For the charm to work, a Secret Keeper had to be chosen, and the secret had to be sealed within their soul. From then on, even if others knew the secret, they could not reveal it to outsiders.