@PotatoZero
A fortress' usefulness is primarily based on it's location. Not whether or not it can be taken. You have real historical battles being fought over the same locations again and again. Jerusalem, Rome, Athens, Istanbul (Not Constantinople). The limiting factor in between the battles over locations has always been logistics. You need to train and arm troops, you need to get food, and you need to get all those things ready.
When this fortress was mentioned in chapter 37, it's said that this is a restart of their annual harassment campaign against it(I think this is hyperbole, but still it happens often enough that it's a thing they have to worry about). So that means they're close enough to get there easily and that they don't have an open ended source of resources to do it. And the way the story mentions it, what usually happens is they break the siege. I would guess, Yelinetta loses about 10% of their siege troops if they have a smart leader that can order a retreat. So, acceptable losses that let you try again. Not a Rout that ruins you for a generation.
Chapter 37, page 9 says, the fort should be impossible to take down in 1-2 months, normally. So that means they've been sieging the fortress for at least 4 weeks. It wasn't an instant win. The bombs, might have been the deciding factor, but it still wasn't an instant win. The Dragon is a separate matter. Yelinetta spent a tremendous amount of resources just getting it to be a weapon they could control. It doesn't seem likely that it's a thing they can just use willy nilly. If it was, they would have used it during the initial assault, not the wyverns. So that's more like saying you got hit by Godzilla/Act of God. It doesn't even factor into the strategic value of a fortress.
And remember, this is a border fort. This is how they project their own power and maintain their control of the nation's border. You ALWAYS have to have it there. Unless they push further into the Yelinetta Kingdom and start taking territory, this place has a purpose. (If that happens, this place still becomes a logistics hub to store and transport supplies/people)
So, let's say 10-20% of the Outer Wall is toast, the inner castle part is still functional. Without Van, you get an earth mage to pile rubble blocking the hole and just station troops as lookout in back, not a full strength wall, but enough to deter simple incursions. Same for the gate, use earthenworks and station a serious regiment. That easily gets you a defensible fort. Not a Great position, but this also means it's not a prime target. Considering their invasion probably took more than a year's worth of planning and resources, they're not rushing back to try to retake a fortress that damaged. So Usable, but not great.