Truth be told it's not much of a defense. I placed one as a test and watched a reasonably low level mob destroy it in like three hits of his fist.
It may seem expensive in the beginning but once you are placing log trees instead of log plants, 200 wood is nothing at all. In one of my games I'm having to either find something to do with my wood or sell it every time that I turn around, obviously I have a lot of log plants and log trees, which is your own choice if you want a lot or not.
What would you suggest? Less wood to build it, or maybe it only costs gold, or stone only or what? The way that I look at it, wood is the primary economy in the game as it's the only renewable resource that can be sold for gold which then buys everything so to me it seems to make sense that it costs 200 wood being how easy it is to get wood in the game.
I guess it also depends on how you plan to use it, it seems to me the initial idea was that it would be used to just fill in an occasional hole that we dug on accident, but somewhere along the line got turned into using it as a primary building block to not only fill in holes but to use for building all sorts of landscape.
