Although it comes packed with the CDDA soundtrack, the Steam release of Powerslave Classic runs on DOSBox. However, one can get a much smoother experience with Steam overlay enabled by using a source port that supports widescreen resolutions and additional tweaking.
This guide was prepared with Raze in mind, a source port that combines various Build Engine ones in a single package with stability and renderer improvements and using several GZDoom systems as a backend. It is a dependable, feature-rich and user-friendly port that can be used to play all the Build Engine games, excluding TekWar and both Witchavens. In Powerslave's case, it is based on PCExhumed code.
SteamEdit, a nifty utility for editing local Steam data, is also needed to enable the overlay while using the port.
Get the latest versions of Raze and SteamEdit here[github.com]and here.[steamedit.tg-software.com]

1) Open the folder titled 'PWRSLAVE' found in the installation directory and copy the subfolder MUSIC and the files STUFF.DAT and BOOK.MOV to somewhere else. These two are the only files the source port needs (the latter being the optional introductory sequence) so once you get them copied, delete all the files in the installation directory to have a cleaner setup.
2) Unzip the contents of the Raze package to the installation directory (while keeping the folder structure intact) and copy back the MUSIC folder, as well as the files STUFF.DAT and BOOK.MOV. The resulting setup should look like this:
A note on the audio tracks
Track 02 is an audio version of the introductory sequence and is not used by the game, so it can be deleted. Tracks 03 to 10 are the audio content of the intermission and the ending screens. It looks like the Raze version (1.1.3) used while creating this guide has a slight, non game-breaking issue with at least two of these - i.e. it plays the wrong tracks for the scenes in question. Thus, it may be better to delete the tracks numbered from 03 to 10 and have those scenes displayed as text-only instead. Tracks 11 to 19 are the actual tracks played in game and in the menu.
3) While Steam is running in the background, launch SteamEdit (it can be run directly from the zipped archive). Find PowerSlave (DOS Classic Edition) in the drop-down list and click on 'Edit Launch'. Erase the lines in the Executable: and the Arguments: boxes of the first launch option and type raze.exe instead in the Executable: one. The external configuration tool is not needed while using the source port so click on the 'Delete' buttons next to the second and third launch options. Click on 'OK' to close this secondary window and then click on 'Save & Refresh' in the primary one. Steam should close and restart by itself.
It should look like this after changing the launch options as described above:
At this point clicking on the 'Play' button in the Steam Library should run the game through Raze.
Note that uninstalling / reinstalling the game or a Steam client update may revert the changes you make through SteamEdit, so keep the tool somewhere handy in case you need to do the last step again.

Be sure to check out the extensive options menu offered by Raze before delving into the Egyptian ruins.
Upon first launch, Raze creates an .ini file in the installation directory (raze-YourUsername.ini) that holds your configuration and can be transferred between the installation folders of the games supported by the port to keep your non-game specific preferences intact.
Apart from the intermission audio issue described above ('A note on the audio tracks'), the automap may also go blank if the game loses focus more than a few times (i.e. Alt-Tab). In that case, reaching the next map or relaunching the game and loading the latest save returns it to normal.
'Welcome to the nightmare of Ramses tomb.'