Sounds like a plan that all works together. We "announce" that there's dangerous stuff in the lower levels (something something sealed evil in a can, horrible world-ending monster thing...), we magnanimously offer to build a fort to protect the rest of the world from it, get that assigned to us as our "sworn duty" by the kingdom; given how well we handled the last catastrophic threat, there isn't a ton of wiggle room for greedy nobles to get a sniff. The kingdom establishes that anybody not-us going in there is committing a crime against the throne...
I mean, that's all best-case scenario stuff, but it's something to shoot for.