I thought I'd go for something more traditional. Here I made a Pyramid. I actually cheated and spawned around 40 Gold Blocks to use for the top but otherwise I dug up over 13000 blocks. 4000 Cobblestone, 4000 Dirt and 4000 Sand. When I told my friend he wasn't very impressed but if he'd be the one who did all that work then he'd have a different attitude. I died only once during the construction of this pyramid. Inside you got two mazes and I'm currently working on a room with special redstone challenges. The first one is a simple speed challenge. The participant is forced to walk on a plate to open a door. The signal is delayed with 2 seconds so if the player will just make it through. He have to jump over a gap as well: Further increasing the difficulty. The entrance is inside the dirt building on the right of this picture and you go below the ground to get inside and then you enter directly into the maze. The pyramid is around 68 blocks wide and 34 blocks long. It's entirely symmetrical.
I also have a Castle that holds 7 floors and quite a few large towers. Pictured coming soon. That one I didn't cheat one bit on.