Um... seriously... if you surround the mill, nobody will be able to use it until the trap is filled in with dirt.
Besides, a lumber mill can cut boards, but it is not set up for cutting those boards into thin strips. You would need to somehow convert the mill into a band saw. I'm not saying it can't be done, because where there is a will, there is a way, however, it wouldn't be easy. You could also just go find a suitable stick for the handle of a shovel without dismantling a spear. Besides, the spear might only be a stick with the end sharpened to a point.
I think 1 iron ore should be enough to make a small shovel the size of a trowel since it can produce 50 nails. 2 or 3 iron would make a nice decent shovel. But you still require a forge.
Filling them back in with dirt was originally my plan, but now that I think of it, it would be a waste of wooden boards to have to do this every time there was an assault. In this case, i'll work on the traps as a future project for assaults yet to happen, as it seems like the work required would be too much before they arrive.
I'll see about some sort of trap system that could easily be armed and disarmed in case of emergency for future raids. Possibly also a fortress we can barricade ourselves inside and pepper the foes with arrows from relative safety.
For the time being, i'll focus on getting equipment for direct assault on the brothers, so scratch those crafting actions then. A viable trap system would need more prep time it seems.
Excellent.
I'm having visions of where this game may go in the future. I'm thinking about having localized maps of the areas directly controlled by the players. It may become too much to have 12 maps for 12 players and need to update each every day, but if there are only a few modifications per month, it might actually be doable.