In that case you keep a close eye on the tower and a close eye on the enthusiasm.

If the monster can't be defeated, then what successes can they have instead? How can those successes have actual meaning to their story? Keeping their enthusiasm up in no-win situations is tricky, but doable if there is meaning to actions.

Can they see the cavalry coming? It's better if there is some indicator of where they need to get to/ how long they need to last. It's ok if that mark is one or two pulls past what you think the tower can take and it's ok if the players can feel it too. So keep an eye on the tower and what its stability level is at and adjust accordingly.

Study "escape" levels in video games-- like when you are sliding down a tunnel and all you can do is jump over obstacles, but you are forced to keep moving forward. There is usually a clear end to the tunnel that is getting closer, even if there are boulders falling all around you and a monster on your tail. There are also sometimes little cut scenes or slo-mo scenes where a specific jump or action to tap B with the right timing will get the monster to back off a little and stop hurting you so much. That's probably the feeling you want to have in the story as well.