So, huh, despite all indications of the contrary, the fortifications didn't disappear. My bad.
Also, I had saved at a critical point, only a few days before the complete freezing took place, and a week before a big siege with the local demonic law-giver happened. The freezing water provided a configuration that wasn't advantageous at all, the moats were rendered rather ineffective, and even after systematic loyalty cascades that would lead goblins to rebel against their own yellow monitor demon law-giver (and man, is the demon tough and strong), I would still be left with many gobbos that would slaughter the military after trolls would take advantage of the ice wall to destroy strategically-placed doors and hatches. One quantic iteration of this siege even had one Forgotten Beast that was discovered months ago but, well, forgotten, make its way into the fort because I wasn't paying attention to the defense device. Damn. And I absolutely didn't want to lose the fort at that moment.
So I tried to do the unthinkable: cheat fiddle with some raws. First attempt: disabling invasions. Well, didn't work at all. Then I tried a small trick to make at least the gobs go "poof" in a cloud of pink smoke. Didn't work at all.
Didn't work, but despite the demon having been killed relatively quickly by its subordinates, the frozen water didn't work to my disadvantage for once, and I found the invaders much easier to kill/catch. End tally: almost all dead, a very few escaped, some more captured, and my captain of the guard lost the use of his foot but gained the awesome title of "The Armored Lions" to compensate. Nothing more.
It then turned out I had modified the raws in the wrong world.
Today's lesson: cheating doesn't pay. Sometimes, the game is merciful toward you. That can only mean you will cry blood soon enough, of course, but you can still enjoy it in the meanwhile.