Really neat idea! I love having build my own defense route. At first I didn't get the point, and I just built a big fence right at the end, but then I realized that I just didn't have enough fire power to deal with everything, which is when I started to build more complex path. I built a route that would go down, then up, then to the right, then to left, both to keep them away from the end for a while, but also so I could place chickens at strategic positions to cover multiple paths.
I also liked having to balance between keeping chickens in the pen to make eggs, but also keeping chickens on the defense to stay alive. Like the other comments said, it would have been nice to have a way to get more chickens though.
I didn't beat the game, but it was still very fun!