Heretic E1M3

Hi

Hopefully relieve the boredom of low message counts :-)

On level E1M3 for Heretic (The GateHouse), there is a secret door in the 
wall opposite  the torch. The wall has a window into an area only
reachable 
once the yellow key is gained and the secret door is further down the 
corridor.


But I can never get this to open. It does not open either side.
Under what conditions does it open?
I have looked at it under editors but dont know what to make of it.

Cheers

Stephen Howe