Welcome to the UCC.
Find a castle with adequate documentation in either fiction or reality, and then try to replicate it in DF.
First up, a few contenders from the Lord of the Rings:
- Helm's Deep
- Minas Tirith
- Barad-dûr
This thread was conceived with the thought of Helm's Deep (which always struck me as a little dwarven, no pun intended). Making a large wall around outdoor farmland (tree and crop), having an intricate system of ramped walls and several gates, a grate over a brook or other body of water allowing for water access inside (add a lever to make it "blow up" and let invaders in), and the large chamber at the top which serves as the entrance to the rest of your fort underground.
It allows for the defense of outdoor farms, treefarms, and water supply, as well as causing dwarves to go inside the "civilian zone" inside the chamber at the top during "stay inside" orders, since it's the only section of underground terrain.
Minas Tirith just had to go in for those folks crazy enough to make a multi-tiered marble fortress with the suicide jump platform to end all suicide jump platforms.
Barad-dûr... Okay, so I've just got a soft spot for craggy obsidian towers that menace with spikes. Bonus points for a magma-moat and stable bridge large enough to walk an army across above it.
Any suggestions or comments? I can't think of any particularly interesting real castles offhand... But motte-and-baileys are always fun, if you can find a way of properly sectioning them. More goal-castles are always welcome.
There's probably going to be a shortage of challenge-takers with the soon-to-come update, but I'm putting it out there just the same. Besides, can you imagine one of these babies in the army arc?