“Destiny Hero – Dreadmaster’s” second sentence is applied if he is Special Summoned by any method, not just by the effect of “Clock Tower Prison”.
“Destiny Hero – Dreadmaster’s” first effect (to destroy your non-“Destiny Hero” monsters and Special Summon 2 “Destiny Hero” monsters) is a Trigger Effect and his other effects are Continuous Effects.
Destroying all your non-“Destiny Hero” monsters and Special Summoning 2 “Destiny Hero” monsters is resolved in the same chain link. Even if you have no monsters that will be destroyed you still get to Special Summon 2 “Destiny Hero” monsters.
If you have any face-down “Destiny Hero” monsters, they will be destroyed, because their names cannot be confirmed.
If “Royal Oppression” is face-up on the field you can activate its effect to negate the Special Summon of “Destiny Hero – Dreadmaster” when he is Special Summoned by the effect of “Clock Tower Prison”. If your “Royal Oppression” is face-down when “Clock Tower Prison” is destroyed, you can chain its activation (flipping it face-up) to the effect of “Clock Tower Prison”. “Royal Oppression” will resolve first, and be face-up when “Destiny Hero – Dreadmaster” is Special Summoned, and then you can activate its the effect of “Royal Oppression” when “Destiny Hero – Dreadmaster” is Special Summoned in order to negate the Special Summon.