ummmm, Talv why the hell would he not be willing? Remember, his lands are the next in line to be sacked if the Duke falls, and he knows this, so you can bet he'll be wanting to drive them off ASAP.
So, What I got on our list of things to do is:
1):SEND MESSENGERS TO EVERY LORD IN THE SURROUNDING LANDS AND TO THE KING! We need to send a warning a mighty army has invaded our country! Include as much detail about the size of the invading force(not much admittedly) and the banners that they were flying.
2)WALLS. NOW.(Also tower). We institute a rotating schedule that has a good portion of people on wall building day and night.
3):While we want to keep disruption to normal life as small as possible, if people aren't building walls, they're drilling. Have the refugees that came start learning the halberd and bow, and sent them and a portion of the town watch top protect the mines. Try to send any refugee not able to fight further up the river out of harms way(though if not possible have them build temporary camps to the north to relieve the crowding)
4):Instruct our fishermen to start stockpiling bows, arrows, and short swords on their skiffs, as well as get a small force to man our felucca and create an impromptu navy(with the felucca as flagship). If trouble rumbles up the river, they are to drop whatever they're doing, dump their catches, clear the decks, form up on the felucca and prepare for battle. As part of this we can craft some simple hunting horns for them and anyone doing sentry duty to sound the alarm with; if they spot an enemy they are to sound three clear notes on it that is to be relayed by all in the vicinity.
4a)As well, we should get some squares of colored cloth and create some simple signal flags for the command ship to relay orders to the skiffs with. As part of this we temporarily appoint someone we trust to the position of Navy Commander(preferably someone who at least has an idea of which end of a ship is which!)
5)If we have the knowledge, construct a ballista/springald or 10, and use them to support our navy from shore/repel invaders from the walls/palisade(or both if we can make enough)