Sunless Spire is an open board game: every design, every graphic, every model and every rule is open for anyone to use, so if you like you make a copy of the game for yourself or e.g. as a gift. You just need to acknowledge this source, and you can’t sell copies of the game without Simon agreeing*.
The files that make up the current version of the game are available here.
*Specifically, the creator Simon Benjamin licenses Sunless Spire under CC BY-NC 4.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc/4.0/