We're at the cutting edge on soon-to-be-released Open BSD 6.3 with SDL2.0.8. I'm the new maintainer of that port.
The only function that's not working is SDL_GetBasePath. Seems to be used rarely and usually can be worked around.
OpenBSD's API/ABI and the libs are updated regularly, so bundled libs usually don't work (short lifetime until they would need recompiling/relinking).
If Tower 57 is written in c++/SDL2, the best (only?) chance for us OpenBSD folks would be an open-source version of the game engine (assets an still be sold). We have a few examples, like Tales of Maj'Eyal (in ports) or Eldritch (port in development)... I know that's probably not the right thing at the moment - maybe keep it in the back of your head...