For the last 8 years now, every couple of years I start a new ULTIMA VII quest, hoping that this time I'll finally beat it, and every time, I encounter some weird glitch or bug that makes the game unwinnable. A few weeks ago I started a new quest and after 55 hours had reached further than I've ever reached before, to the extent that I was ALMOST at the end of the game (I partly chalk it up to avoiding use of the notoriously buggy magic carpet entirely)... I had done all of the "follow the trail of the Fellowship members" optional main quest, had started getting ready to do the "destroy the generator" main quest, had completed the FORGE OF VIRTUE add-on, before getting back to the generator quest. So I entered Dungeon Deceit, started moving north through the tunnels, when suddenly, the game just froze. I couldn't even access the inventory or save/load screens, and the only thing that didn't freeze was the music/SFX. Curious, after CTRL+ALT+DEL my way out of the game, I reloaded the save file and decided that maybe I should try to go after the Sphere generator first. So I sailed back to Britain, began heading north to the dungeon, when suddenly... the game froze again (slightly north of Lord British's castle). So I went onto Google and looked at a map of the game, and saw that the spot where the game froze at the Dungeon Deceit was almost on the exact same line as where it froze north of Lord British's castle. It's almost like there's this invisible line that exists in the game now and if I try going past that line, everything freezes. I loaded up some of my earlier save game files and this does NOT happen, which makes me wonder if the recent save game files have somehow gotten corrupted in some way? It's very bizarre!

I'm playing the original game on DOSBox that I purchased through GOG, NOT the Exult version, if that helps...

Please attach / upload a copy of the affected saved game (GAMEnn.U7) here; it could be helpful to someone now or in the future to troubleshoot this issue.

Have you used the cheat screen and/or teleportation map at any point in the game?
Not judging, but this could lead to a possible clue.
If you did, which cheats did you use?

No, no cheat screen or teleportation map... I don't even know how to activate such things (hell, until now I didn't even know there WAS a teleportation map).

I went back to an earlier save file from the Isle of Fire quest, which was at a point where I forged the Black Sword and completed all 3 tests, but before I had completed the side quest total (that is, sending Exodus to the void and going to tell Lord British about it). I left the Isle of Fire, sailed over to Dungeon Deceit, and now find that I no longer have the freezing problem! So my best guess is that for whatever reason, completing the Forge of Virtue quest seemed to be what triggered it?

Weird. I hope the problem won't return and that it'll be smooth sailing from here on!

H'mm, the mystery deepens. So I made it through Dungeon Deceit and destroyed the Tetrahedron generator. Then I went to the dungeon with the Sphere generator and destroyed that one as well. "Two down, one to go." But I received a nasty surprise when I used the HELP spell to return to Lord British's throne room... all of the walls in the world had vanished! I've heard of this glitch occurring in other contexts (like winning too much money at the casino in Buccaneer's Den), but I'm not sure what must have caused it here. Anyway, I exited the game, reloaded the save file, and after destroying the sphere generator again and picking up the prism I went to save, and received another nasty shock: a message popped up saying how the "Ultima VII self-check reveals a corrupt game blah blah blah" box. That message pops up if I try to save in any of my save file slots. Very odd!

The disappearing castle walls is a famous bug indeed. It does generally happen when the player creates so many items (winning money, as you said, but also hunting for food, buying lots of stuff, etc) that the game can't store them anymore, and weird things like that start happening.
Perhaps there are other reasons too. The folks over at Exult probably have a better explanation for it.

Speaking of Exult, have you tried playing the game with that? In my opinion it's a safer way to play the game.

Not sure what the corrupt game message is all about. I've never seen those before, not even in vanilla. I guess your progress is once again halted then? :(

I've heard a lot about Exult but I wanted to beat the vanilla original version of the game first I guess.

The only other time online I've seen someone mention this "corrupt game message" is in this GOG thread here (though in relation to the Cube generator): https://www.gog.com/forum/ultima_series/ultima_vii_issue_with_game_corruption_during_cube_generator

It seems what triggers the vanishing walls/corruption message is destroying the Sphere generator.

Well, problem solved, kind of... I went to an earlier save and did the Cube generator first, with no problems. Then I went back and did the Sphere generator, and this time there were none of the disappearing walls/corrupted save games bugs. I just beta the game a few minutes ago! Anyway, thanks for the advice and suggestions.

Welcome. I hope you'll manage to finish the game!

Yup, I did finish it last night