Thanks to Youtube who proposed me a video by Retro Theory called Amstrad PPC512 ISA Expansion Board XTIDE Network Sound PPC640 where is he testing his ISA expansion board using the 2 connectors at the back of the Amstrad PPC 512 or 640.

Check the description of the video for the github page with the pcb files and the components list.

So in this video he is testing an XTIDE, memory card (to 640 Kb and some UMB), sound card using the unisound (universal sound driver) and a CGA/EGA card. So he launchs Prince of Persia with OPL2 (sound blaster) and EGA.

More Amstrad content should appear on his youtube channel later as states in the commentaries, so stay tuned and subscribe to his channel like I did.