This is a design mistake in the original Hexen script (if you pull the levers in both Guardians of Fire and Steel without visiting the Seven Portals map in between, the secret level remains inaccessible). Fixed in KMOD 5.3 with a modified ACS for all three affected maps.

Steps: 1. Solve the Guardian of Steel puzzle. 2. Step on the center platform to get the message “Stones grind on th Seven Portals”. 3. Go to the Seven Portals. Looking at the script code it checks if it was ran twice but I didn’t found another place where it is called. Maybe I’m just wasn’t thorough enough.