Yeah I think castles would be great too! Dams might be a good place too, because there are only narrow pathways to get in, easily dependable. as a last resort you could just jump in the water. Also water is abundant there, I sure they have food storage or if not I'm sure you could find some food.
Note: Also because it is on a river and there is a lake there should be an abundance of water.
Gardening supplies are also readily available all over the city and surrounding parts. Or use the soil you can find yourself using the sediments from the river or go to a nearby mountain and dig some good stuff yourself... just ideas but I am not an expert on the matter. Preserving food supplies