Assuming Roxbury hasn't been taken over, I have a few simple ideas that can be quickly implemented to bolster the defenses.
1. Reinforce the walls with mud. We already know the fields are bogs, so we have a ready source of mud. If we pack mud against the inside of the walls, it would seriously improve the walls' strength.
2. Sharpen the tops of the wall and/or add sharp stakes to the top of the wall. Obviously, the orcs could still climb over the wall, so we need to impede their progress.
3. Have the villagers craft watch towers or platforms atop their homes. These should be simple to build, assuming we have a ready source of lumber. In addition to this, if we can arm some of the villagers with bows or spears, this would be a plus.
4. Ready fire pits inside the walls. Essentially, dig ditches inside the wall and load the ditches with flammable materials (dry wood, oily rags, etc). We only light them up if the orcs are actually coming. What we can do with the fire pits? Well, we can give our archers flaming arrows or, in a worst case scenario, light the walls on fire to keep the orcs at bay.
I doubt we'll have the time to fix the defenses up before the orcs arrive either. 
