Doomcaliber Knight

(First time posting. I read through the FAQs and I think this belongs here. If not, then I'm sorry and you can delete this)


The Doomcaliber Knight has the following effect "This card cannot be Special Summoned. When an Effect Monster's effect is activated, you must Tribute this card. Negate that effect's activation and destroy the monster."
I'm a beginner, read the rulebook (but English is not my native language, so there might be something I misunderstood) and this came up in a duel:

I had the knight in face-down defense position,
my opponent activated the effect of one of his monsters and, thinking that I had to do it, since the tribute is not optional, tributed my knight and wanted to destroy his monster.
My opponent, a self-claimed long-time duelist, (can't remember his name) said that I couldn't tribute my knight since it was face-down.
I disagreed.

Who was right in the end?
Please answer me this question.

Sorry to tell you but the other guy was right.

The effect wont apply if Doomcaliber is face-down.

face down monsters do not have effects except if they state they have effect xy while face down (like big shield gardna for example)

Your opponent was right. You cannot tribute Doomcal for its effect when it is face-down on the field.

He was. True, Doomcal' effect is mandatory, but it only applies while face-up.

okay, thanks for the help