Helpful screenshots are on the first post in this topic.

You only have one chance to find secret rooms 1 and 2 - at the beginning when Marin is with Lord Merryll. After that part's over, you can't go back. The mouse doesn't find these rooms for you; you have to try walking into all the walls in the cave until you find both secret rooms. (I can't quite remember, but you may need to press space bar as well to find these two rooms.)
You can go back and look for 16 and 19 - as long as you haven't dynamited the orcish fortress. Go back down the orcish lift and explore both inside and outside the fortress. Use the mouse to look for sneaky places where you can jump. Secret room 16 can only be visited once.
Good luck!