Sealed Doors (Creation of)
Several sealed doors exist in 2300 A.D. bearing the mark of Zeal and requiring energy from the Mammon Machine to open. Who constructed these doors?
In all probability, Belthasar created them to safeguard and leave treasure for the expected saviors of time, Crono and his group. How he obtained the foresight to do this is unknown, but he does demonstrate a knowledge of the party and their quest when speaking of Death Peak through his Nu assistant. The only problematic aspect of this answer is that Belthasar would have had to journey long and hard through 2300 A.D. to reach all the domes he constructed doors in. Nonetheless, the Nu could have conceivably done it in his stead. What seals it as Belthasar is this important tidbit, left out of the NA translation:
[Man] There's an old man who built a dome near Death Mountain and keeps watch like some odd hobby.
Belthasar built the Keeper's Dome; it is likely he can build a few doors, too.
From: Common Questions