If you are using Windows or DOS (in case you want to try it on an old laptop from the 90s) then you can download Petra099n.zip and run one of the executables inside. For Linux download Petra099nLinux.tar.xz. If you have a Mac download Petra099m.dmg - it is a slightly older version, but there weren't any Mac-specific changes in 0.99n anyway.
Assuming you are using the Windows/DOS archive, the executables you may want to try are:
- glpetra.exe - Windows: this will give you the OpenGL HW accelerated version
- sdlpetra.exe - Windows: this will give you the software rendering version but wont support gamepad
- gdipetra.exe - Windows: this will give you the software rendering version and supports gamepad, but it will be choppier
- d3dpetra.exe - Retro Windows: Direct3D 7 HW accelerated version for old Windows PCs
- winpetra.exe - Retro Windows: DirectDraw software rendering version for old Windows PCs
- petra.exe - Retro DOS: DOS version for old PCs
If you are not sure and just want to check out the game and your laptop is not older than 15-20 years or so, you can run glpetra.exe.