How open is stronghold/fortress? As open as you need, if you view past challenges, except for the ones that had a very specific map to follow, most challenges are WIDE OPEN. As open as your creativity allows.

If you want to make your stronghold the bunker in "Lost" tv series because that's the way you want to go - then its OK.

I am seriously thinking about a Dwarven mine fortress under assault by Drow, and everything is underground.

Get as wild as you want, as long as you can justify what you selected as a stronghold, it should work fine for the challenge.

GP

PS: I thought it was going to require using "placeholders" for maps, had the challenge been about a printed map. (its not, I know...)