Realms of Arkania: Blade of Destiny

Secret doors are scattered in almost every dungeon in the game, but your characters need to have a decent level of perception to actually notice them. So a good tip is to save every time you run into a dungeon. (dungeons meaning caves, abandoned buildings, mines, and other locations you find as you travel).

Note: if you find a door, but it won't open, just step away and then come back to it again. It's a chance thing. Eventually, even at a low level, it will open. And yes, I'm pretty sure you need to actually open it for the door to count.

If you find a door and successfully open it, you can reload that save and keep opening the same door. The game will still register it. Otherwise, you can also just keep exploring the world until you run into more as there's an achievement for visiting all the dungeons anyway. Either way, the achievement will pop once you've discovered your 5th door.