[Edi]

Activating the Colossus costs only one commander turn, or more precisely, it costs 3 AP. That'swhy moving into teh square was 1 AP, activation 3 and only 2 AP left for movement next turn, which you can interpret from the narrative.

Any commander can activate it. It cannot be controlled, because it can randomly appear even right next to a starting citadel. If it were controllable, it would be an instant "I win", which is why it was changed to being a stupid unit that cannot be led.