Sanctum Island
After some time, you arrive near the island with the santum on it. Whilst approaching the island, you once again think about what Tabitha told you about this particular sanctum. It's led by a witch of the Sisterhood known as Magistrix Yvonne Lebraux who is specializing in studying artifacts. The sanctum is located on the top of a hill on a small island, below it there's a small town where servants of Magistrix Lebraux and others from the Sanctum live. Visitors from the outside are mostly invited, those being merchants specifically contracted to supply the sanctum etc. Sometimes, complete strangers decide to visit the island and they are permitted to stay if they have peaceful intentions, but they are watched closely. So if you just dock there, you can be sure to be kept under a watchful eye, which will likely complicate your efforts to raid the sanctum. Tabitha also told you that Magistrix Lebraux is always looking for worthy apprentices, so that's a possible way to obtain access to the Sanctum proper, though only women are accepted by the Sisterhood.
The description fits this island. There's a large, forested hill in the center of that island and on top of it is a large, ivory tower with several smaller buildings of the same color attached. This must be the sanctum. It's encircled by a tall, stone wall. There's a path leading up the hill to the sanctum, up to a large wooden gate.
Under the hill atop which rests the sanctum, there's a small port town, you can guess it would have to have at most 50 inhabitants. There's a tavern, a general store and a shop with curiosities but that's about all there is to do for the visitors. When Memelesi docks in the small port, people come to gawk at you great vessel. Large part of your crew quickly leaves the ship to spend some of your newly-earned coin at the local tavern. One of the locals, dressed in fine silks and obviously someone important, approaches Orlov and whomever else happens to be around at the time. If they come down from the ship to the port.
"Welcome to our island. What business brings you here? And will you stay here long?"