So on level 1 I've found the secret room near the entrance and managed to open the room in the middle.
And I don't know what to do now. The door on the left of the map won't budge, so do the doors on the right .
Help!

There's a switch in the big room in the middle which opens the little room directly next to it in the west. In this room you can find a switch which opens the door for the room in the very west of the map. There you can find switches for the doors for the room in the north-east.

Brilliant indeed! Thank you!
And I got through the two doors in the east. There I found a hidden brick when pressed open a slide door. In there are a few coffins and another door which I can't find a way to open. Any idea?

I actually don't know how to open that door, I'm sorry. But you don't need to open it to proceed downstairs.