This is a castle I am making using mostly nether blocks. Its not done , but heres what i plan to to:

*Fill ground with lava (using mc schematic)
*Add a tower at each corner
*Add barracks
*Add bedrooms
*Add a throne
*Add a large dining hall
*Walk ways above the lava of netherack
*Add netherack glaciers
*Add a dungeon
*Court yard, with simulated nether plants

If theres anything else i should add, notify me with a PM, or in the comments :)