A 15x15 tower that extends from sea level to the top of the map world. 16 floors + plus the rooftop (which has swimming pool at the very top, extending into the floor below with glass.
On each alternating floor starting from floor 2 there is a walkway extending around the tower complete with battlements. The roof has the same battlement/walkway arrangement, but has a second battlement walkway slightly lower than the first.
Each floor has redstone circuitry extending right around the room, lit by several redstone torches. The battlement levels also have this circuitry following around the pathway.
There are 2 3x3 type spiral staircases in the rear corners which extend to the very highest point of the tower.
Hundreds of torches line the inside and outside of the tower ensuring every inch of the tower is well illuminated at all times.
There used to be a small forest of trees surrounding the castle, but I eventually cut them all down and replaced the soil with cobblestone to create a courtyard. The courtyard took up more space than was originally available, so the tower now compromises the entirety of an island unconnected to the mainland. The courtyard has battlements too.
Under the tower is an enormous mine extending to bedrock, just above which I am currently constructing an enormous monster trap (100x100 in size, all carved 10 or so squares above bed rock). The trap itself is intended as a drowning trap, but I'm considering how I can implement a lava blade into the design for efficiency reasons.
Outside of the tower I have a glass underwater fort accessible by boat that I was constructing before I started my monster trap. At present there are 2 levels, the first 10x10 and the second 20x20 with an adjoining 20x30 long hall. I'll continue construction after the monster trap is complete.
I also have a few smaller projects including a greenhouse where I grow wheat, a cactus maze, and some experimental logic gates that I have recently started (I recently got a random logic gate to work, which I'm quite proud of).
In real life I live in a small room and even if I had piles of cash, I'd just buy a nicer small room with better stuff in it.
So... My Minecraft base says that I like to waste time on a computer I guess.
Not much more.