Robert Maxwell wrote:

The walls will eventually be about as high as the towers on the corners. Yeah, I need a lot of cobblestone...
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I'd like to help you out, having stockpiled six large chests full of cobblestone, but it's SP.
I haven't played much Minecraft lately, but I have stockpiled a nice amount of resources (two large chests full of stone and 120 blocks of Iron

), so I'm set to build stuff now.
Then again, I'm tempted to rejoin the SMP server I was playing on before the Halloween update (they reset the map at the time and I started my SP world). They've built some impressive stuff. I especially like the Colosseum, it's still under construction but already very impressive:
http://www.youtube.com/watch?v=Gkl81iX9j-Q