When Prince Ambrose decided to follow his sister's advice and go straight to the capital, he was anxious at first even though he had fully agreed to go along with the plan. The green river was impregnable, dangerous and huge. He remembers when he was a boy, he used to travel with his friends just to see the green river.
Some of the villagers nearby said the river was full of crocodiles, and some even went further and claimed they had seen huge snakes in the river, but throughout his years as a teen, he never saw any of the things the good folks spoke about.
It was winter and the prince couldn't have picked up a better season to cross the green river, during winter the river turns to complete ice hence this made it easy for him and his men to pass through. Considering they wouldn't hasten the horses or cross it in bulk, the ice was thick and strong to withhold all of them.