To activate the cheats you need to press and hold SHIFT + ALT + A at Main Menu. The cheats will be activated until you close the game. List:
• ALT + X → 100 popularity and 1000 gold.
• ALT + K → All buildings become free.
• ALT + D → DEBUG Mode.
• ALT + Q → Takes screenshot (works without pressing the keys at Main Menu).
• ALT + C → Unlocks all campaign missions.