Saw RTCW (Return to Castle Wolfenstein) in the Debian repositories, can't seem to download in terminal:
Tried iortcw, didn't download either and iortcw is based on ioquake3 which is downloadable:
Without the RTCW repository you would have to build iortcw yourself to run on the RPi. To run iortcw you will need a copy of RTCW that is installed on a x86/x64 computer, you will need to copy over the install folder to the directory where you compiled iortcw, here is the link:
Read the Raspberry pi readme to install, the game runs at full speed on a RPi3b+ at 640x480. Not sure if multiplayer works or not, only played the first level.