Hi!
I've actually got a few different ideas for Yarntown that I'm not sure I won't get to someday, so I don't know if I'm ready to just give the reigns away ;-)
What I can't stop you from doing, however, is opening up Yarntown in the Solarus engine and seeing how it's code works, then creating your own demake of Bloodborne. I've even written thousands of lines of open-source code for the engine for folks to use that lend themselves to this kind of game (and TBH, are written better than Yarntown's code), and I'm selling the tileset that I used for this project.
If you want to make a similar thing, open up the engine and start playing around, the Solarus discord is a great place to ask questions about how things work and it's got a long series of tutorials on YouTube.