Promisance source code and Nuitari's Promisance is under the GPL licence.
It's full text can be found here.
Basically you can take the source code to the game and make any modification you make, as long as you put them available for download.
If you do not, you are in violation of the licence. You must also maintain any Copyright notice in the code and the program, but you can add yours to them. You may not, under any circumstances remove or alter any of them!
All images on this site, including those in the game, are © 2000 Nuitari & Snell Productions. You may not use them.
The game guide and all its images are under the GPL licence and are © 1999-2000 BW Computers