The map is procedurally generated. Map layout, Number of Secrets and Secrets Locations will vary for each playthrough. One Secret Rune can be collected only after you have completed the entire main campaign. So, in your first playthrough you may not be able to collect all the secrets. Check the video at the end of the page if the written walkthrough is not clear.
Just after the level starts, move a little forward and get down the stairs. Turn around and you’ll see 2 shields hanging on the wall. You will be able to interact with one shield to open a secret entrance. Inside you’ll find a map to unlock Underhalls Secret Mission.
Secret 2 (Can be collected only after completion of Main Campaign)
During the mission you’ll reach a courtyard where you’ll face a lot of enemies. After defeating the enemies, to move forward you’ll need a golden key. From the locked Golden door turn right and go up a small stair flight to find a switch on the wall. Intereact with the switch to open a secret door. Go inside and collect a Secret Rune. Collecting 9 Runes will unlock a Secret Cow Mission.