Forum:Defender?

Situation help: I control a Torrential Tribute... My opponent controls 3 monsters (2 spellcasters face up, 1 is a Defender, The Magical Knight, and 1 is a face-down defense monster.... turns out to be a spellcaster also), and a Magical Citadel, and with only 3 spellcounters on the entire field... My opponent summons a monster, I activate Torrential Tribute... can my opponent activate Defender's effect and block his 3 spellcasters (face up) from being destroyed?.... Does the face down monster get destroyed?.... and is he allowed to use the effect?