The Sealed Door

We're still traveling down the river, but the weather is slowly getting better. We stopped again at a town while we were sleeping, and a whole bunch of the passengers got off. It's still crowded, but not so bad now. Magus Celon spent all the rest of the afternoon and evening in his cabin, and at dinner Mr. K told us to leave him be. When the magus saw that ruined old tower, he went white as a sheet. I asked Mr. K about the ruins, and I expected him to tell me not to worry as he watched Gloria and I eat our bread and cheese. Instead, he told us a story.
As he told us about it, something softened in his face. He's such a cold person, it's easier to think he's not human, and I assumed that he would be a bad teacher, but this was different. He doesn't deal with people well, but he's passionate about history. I'll try and reproduce the history of this place, as neatly as I can.
The tower has existed for as long as anyone can remember. Records of the site, according to Mr. K, date to the beginning of the Second Age, and even then, the tower had been ruined. Whatever purpose it held was lost during the Fall of the Old Gods. That means it was probably a temple or some other religious building that was forgotten by the Risen Gods, but the truth of the matter is still unknown.
The interesting thing about the building is that no one has ever fully explored the structure. The stones above, which are stronger than they appear, and still bear mysterious enchantments, eventually lead the visitor to a single door, marked with the rune of the Prime Magister! This door is taller than two men, and just as wide. It is carved of stone, all in one piece, and doesn't have a lock or a keyhole. And no one in the world has ever been able to open it. There are many different theories about what is there, and what could be behind it. That would explain why everyone was so excited. The Prime Magister invented most spells and magical artifacts. He's a hero so ancient and famous that he commands almost as much respect among humans as the god Hattori himself! They say that he hid his greatest secrets, in his grimoire, away when he died. Somewhere, out in the world, his canon is waiting to be rediscovered.
Mr. K would have liked to have stopped the boat to explore, but with that many passengers, even day-trippers paying with copper, the captain wouldn't do it, so he made what notes he could. I wonder if Mr. K has a bit of an adventurer in him?
As for the reason Magus Celon became so upset, even Mr. K doesn't know. He just said that elven memories are long, and their moods are changeable. He advised us to leave the mage be and enjoy the river trip. Even though the "mortal" histories of the first age are mostly lost, it is said that the elves considered the Prime Magister a dangerous villain. No one outside the Green Kingdoms knows why, though. The elves know more secrets than the gnomes, but they keep them close to their hearts.