You are very limited in what you can accomplish in terms of sabotage. The only thing that comes to mind will cost you a lot of friendship points with various lords and may end up getting you kicked out of the kingdom.
What you can try to do, is to sow dissent within your kingdom by questioning the success of your marshall. If you can somehow get him evicted and become the marshall yourself, you can command as many lords as you can into attacking strongly fortified positions or just to lead them to the other side of the map while your castles/towns/villages are sacked.
However, losing towns or letting villages be raided while you are the marshall will cost you a LOT of friendship points and will get you evicted in short order.
The only other thing you can do is to talk to each lord and try to send them in wrong directions (I suggest you ride to so and so location, etc).
As for fortifying your own town, you cannot, but if you have certain mods installed (I highly recommend Floris mod), you will be given to option to swap fiefs and/or send out patrols to defend those villages.
As a last resort, since you only have a single village as a fief, you aren't too attached to the Nords and you can simply defect and join Swadia instead to help them out.