Does Imperial Decree kill units on the Frozen Fortress battlefield during my turn when I play it and there are enemy units there?

No, playing Imperial Decree during your Main Phase will not kill units already damaged by Frozen Fortress that turn.

Frozen Fortress deals its 1 damage at the start of the Beginning Phase — before your Main Phase. Imperial Decree only triggers on damage taken after it resolves. It does not look back at damage events that already happened earlier in the turn (FAQ #11279, #10843).

However, if you play Imperial Decree on your turn and it remains active until the next Beginning Phase (including your opponent's), then when Frozen Fortress deals 1 damage at that time, Imperial Decree will trigger and kill those damaged units (FAQ #10596).