Is today the last day, or is that the 18th?
either way, my submission is just about ready

Well, I'm probably not going to work on mine much more, so here it is so far:
This is the home of a mob from another unknown Minecraft dimension that landed in the overworld. Here is its picture:

First, notice that there is a brewing stand on its head. That is because brewing stands are found in the overworld and the end. They are found in end ships if I remember correctly, and they are found in overworld igloos. It would be inaccurate to say that they are not also a nether block, because the key ingredient for making them is a blaze rod which is only found in the nether. Here is a far-off view of the strange entity's home:

Here is a closer view:

Notice that there is obsidian in this structure. That is because it is very hard to break and it is found in every dimension. It is found in the pillars that the end crystals are on, it is found in nether portals, it is also found in the overworld when water and lava combine in the right way. Here is a view of the main function of this structure, the nether wart farm:

Now I will show you how it can defend against mobs. The next few images will show the security that is here: First off, I will again show you the first image. If you read on, you will understand why.

I included this image again because if I lure mobs too close to this, then they will ride the boat. If a player, a zombie, a creeper, a skeleton, and probably some other mobs ride that boat, they will get suffocated by the gold block.
Here are some more images and their explanations:

There are grass path blocks beside some of the lava. Because grass paths are not a full block tall, if you stand on the grass paths too close to the lava, you will burn. This is very effective against mobs. Also, lava is in 2 of the dimensions. I know that blocks do not have to be in both 2 dimensions, but I just think it's interesting how many of these kinds of blocks you can find. There is also one block that is in just about every build that is essential that is in every dimension. Here is another image:

This is my anti-monster door. If a zombie or creeper, or even a skeleton walks through that web, I can either kill them with my fists or whatever weapon I have on me or I can lure them on top of the iron trapdoor, then hit the stone button to make them fall in lava, and then they die. It is fun to do, in my opinion.
I think that this just about sums up the defenses in this build.
Now I will show you some more blocks that are in multiple dimensions that are in this build:

This chest may look out of place, but where am I supposed to store my brewing supplies? This is a nether wart farm, and in the middle on top of the strange mob's head is a brewing stand. Also, chests are in every dimension. They are in end cities, nether fortresses, and almost every other natural structure you can think of, with the exception of a minority. Also, here is another image of how I use chests well in this build:

This is me examining the contents of a chest that I hid underneath one of the iron trapdoors in the lava.
This is a very useful way to hide your valuables.

This shows some of the glowstone in this build. If you think about it, it can be found in both the nether and the overworld. Witches drop it, and you can mine it in the nether.
Finally, here is a block that is found in every dimension and is used in almost every build,
The air block.

The Shapeshifter's Castle

It looks like an end fortress, correct? Go through the door and see what happens...

It's made of blocks from Overworld and End

These are the only screenshots I have in my phone right now, I will post pictures of the garden and the interior when I'm back!

What's on the inside

@SovietSuperman_ What's on the inside

@Cileklim
These are the only screenshots I have in my phone right now, I will post pictures of the garden and the interior when I'm back!

When will the submissions be judged?

End fortress from the outside, sea temple from the inside. Built from scratch in CraftyMynes.

Inside:

Roof Garden:

I'd like to say thanks to @Eberk91, @Tez1010, @FriendofP and @Naidae for supporting me in every way they can. They gave me an infinite number of supplies, motivated me, helped me with ideas. I'm pretty sure I couldn't even put a single block without them.

Man, that build turned out awesome!! :D

I think we have a winner!