This is the guard tower part of the wall system I have around one of my castles on my server. I've seen some great towers but only a few that link up to an actual wall that is also posted here.
The modular wall section is also posted on this site as well as a bend or corner piece for the wall. The wall connects to this tower.
To download the wall and corner pieces: http://www.minecraft-schematics.com/user/nexev/schematics/
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MINECRAFT-SCHEMATICS.COM