Thanks for your answer.
My mistake, I should have tested it more profoundly.
However, after making a network game with 2 computers and tested Orthanc Guards against Gondor soldiers with heavy armor, I must reach the conclusion that in fact there is no difference whatever place Saruman is on: garrisoned in his citadel or in the field.
The 50 % damage against everything and 50 % aditionnal against units with heavy armor simply don't work, it's not the reverse way as I thought, but simply doesn't work for now.
I tested it with 2 different Orthanc Guards against 2 differents Gondor soldiers fully upgraded.
One fought when Saruman was garisonned in the citadel, the other fought while Saruman was in the field. I put the same stances to everyone
I saw absolutely no difference in the way the combat occured and in the final result :
the Gondor soldiers took 4 hits each time before dying, and they won, getting each time to level 4.
I'm sorry for having investigated it but when a concept is very nice such the Orthanc Guards are, I often have to assess they work OK, it's stronger than me.