- You can move only on your turn, and you can cast only on your turn.
- When an enemy comes to you on their turn, you can engage, and you can cast. This includes when they are attacking your city.
Why could the Archons cast the DDR spell? They were part of the Coalition and it was now technically their turn. But they could not move because all of Charlescomm's units' move went to zero when Charlie ended turn. Move and hits are restored to full at the beginning of a side's turn, and (re)forming an alliance does not count as starting a new turn.