This is a palisade: (edit: not as big an image)
This has come up before a couple of times (or so the search function tells me) but not recently and it's never been discussed in detail.
Briefly, I'd embarked in a terrifying above ground area with a double aquifer layer in a grassland area with a moderate amount of trees, and lo I thought to myself: "There are monsters coming, I'd best build a quick defensive wall!" and decided to work building a quick wooden wall.
Except it wasn't that quick, I'd gotten about a third of the way there when undead horses killed me off. In fact it would have been quicker to pierce the aquifer layers and mine out some stone than to do it with lumber. Effectively meaning a stone wall is quicker and cheaper than a wooden wall.
So my proposal is this: Allow the construction of a structure similar to the way paved roads are built, that costs 1 log* per X squares, probably 3 or 4, to enable the player to make quick defensive structures when stone isn't available. *They'd be fast to build, (much faster than roads) maybe more like workshop speed and would be constructed and deconstructed the same way as a road (single entity in rectangle shape), built by carpenters and wouldn't need to be designed. Possibly there could be a similar metal version (fence?) that might be somewhat tougher and built with metal bars.
The differences between this and a normal wooden wall is threefold:
1) It's destructible, in the same way a bridge or road is destructible.
2) It doesn't create a floor above it.
3) It lets liquids pass through.
This means while it's effective against most beasts and ambushers, it'll only delay sieges and larger creatures for a short while. Plus the lack of a floor and the inability to stop liquids means it doesn't shunt wooden walls into obsolescence.
I think the coolest thing about this would be that it'd mean you could set out and build a hasty defensive structure that provides limited protection on short notice.
Anyway, what do you think? I'd love to get some feedback.
*denotes edits to include feedback.