Danke für Eure zahlreichen Kommentare und Hinweise :)
Der Guide ist nicht als vollwertige Lösung gedacht, da ich denke, dass es keinen Spaß mehr macht, wenn alles im Vorwege gelöst ist.
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Thank you for your numerous comments and suggestions :)
The guide is not intended to be a complete solution, as I think it's no fun if everything is solved in advance.

There is an extra secret room on Level 3. If your going from room 17 to 18 check the right hand wall for a hidden stone button. On the picture the button is right next to the number 17 at the T-Section and the secret room opens behind you. I have a picture on my profile which shows the location of the room. I got a reinforced mail armor with +3 armor and +20 health.

@WhyNotZoidberg For me it was just empty and there are more of these across all maps. I'm guessing they wanted to condition players to associate these niches with loot, just to sometimes have them empty.

The minotaur in level 6 drops a beatifull ring

There is an extra secret room in level 4. Near the blessed sword room, a suspicious torch on the wall can be turned to open a passage with a fountain and a serpent dagger.

Has anybody figured out if the empty niche (11) on level 2 is good for anything?

Level 3, in the room with the level boss, you can find a usable torch on the northern wall (secret room with composite bow)

Just managed to finish the final level by a smidgen. One last player almost dead. Won't spoil it for others. Maps a terrific help.

Level 4, on the way to S9, on the right wall you can find usable torch.

Found some more on another run that I did. On level 6, 4 spaces above where it says there's a mana potion (1) there's another mana potion, in an alcove in the wall. What's more, from that location, go west, west, north and you'll see some Golden Greaves on the ground (light armor).

On level 7, at number 14, you only need to flip the third switch on the wall (counting from the east) to open the secret area where the 2-handed axe 'Snaga' is located. You only need to flip the third switch (counting from the west) to open the regular door. All the other switches toast your hide with a fireball.