I’m still somewhat new to the game, just got to plains and have cleared out some villages but haven’t even heard of a sealed tower, is this something you made or is it a naturally spawning tower? It’s really cool either way!
This is a Challenge that you unlock after speaking to the 2nd trader Hildir. (found randomly in meadows biome like you find Haldor in black forest) there are 3 challenges in total, the sealed tower is one of them. Enjoy the game!