One of the most challenging pieces of planning on this was the crenellation spacing. If you lay it out wrong you end up with them uneven, or half of one at each end, or in extremely severe cases, half a crenellation extra.
The gate was the one disappointment of the MOC for me. I was hoping to make a mechanized gate using gears and cool stuff, and I got a partially working one, but in the end I got annoyed with it and used a simple pin system. Much like you'd find in an official set.