I'd like to present to you the culmination of 2 months of work (I wasn't playing hours on end every day, which is why it took that long) a fortress known as Mestthos Deler, which translates as "The Citadel of Steel."
First and foremost, the wall.
It is approximately 5 tiles thick and 6 z-layers high, it is made entirely out of steel blocks (more then a thousand, most likely) and is designed with one thing in mind; to funnel invading armies and ambush parties into a narrow, easily defensible chokepoint (the last image in this dump will be an overall view of the top layer so you can see it.) It's tall enough that marksdwarves can fire at enemy sieges whilst being protected behind their fortifications, while it's designed so that invading goblins would try to get up the ramps first, being easy prey for my swordsdwarves.
The second part of the fortress, and perhaps more flashy, is something I like to call the Golden Spire.
Golden walls make way for copper floors, the thing was meant to be partially modeled after Isengard from Lord of the Rings, with the spiky spires on the top reminiscent of the top of Orthanc. In total, the thinnest part of the tower is 9x9, while the livable parts +33 z-layers above the base of the tower are 13x13 rooms. The entire tower took 2800 bars of gold and 2400 bars of copper to make.
This is the top of the tower, the blue is, obviously, Adamantine. I had enough Adamantine to try a Lincoln Memorial style seat to Armok, but the vast majority of Adamantine was already directed towards furniture and military matters and I wanted to make the seat for a different megaproject fortress. In all, from base to tip, this spire was around +41 z-layers high with the possibility of being even higher, if I reclaimed.
This is the overview of the entire fortress, the final result of over 13 in-game years of hard labor, to it's left and right are Khazad-Dum style hallways that stretch 5 z-layers into the mountainside. The roads are made out of silver.