Pretty much what Remixer said. I've imported a custom skybox (skysphere) model under the path "Environment\Sky\DalaranSky\DalaranSky.mdx", then created a trigger in the map initialization to set the sky to dalaran sky.
There are a couple of obvious problems when doing this for melee maps however. The fog of war will apply as a texture to the alpha tile and screw its transparency, so anything below it that is covered in fog will be invisible until you reveal it (luckily the skybox is unaffected). That's why I can't make hanging gardens and huge pillars downwards, which is a shame.
EDIT: Here's my 4th WiP, the outer ring is under construction

I can't help but wonder if I've made the map too big and complicated for 2 players...
, when you uploaded your map to have help with your wc3 file corrupted, I could access to you map and it secrets: muahahahaha!!. just kidding, if you want to know my opinion since you said:
"can't help but wonder if I've made the map too big and complicated for 2 players"
even if is a 160x160 the players are not in the corners so distances can work (I think), a chrono timing how much time do you need to reach X from Y will help too.
Try to do this, copy paste all units (buildings) and paste them in a new 96x96 map. If they fit your are fine.
I am working in a 128x128 and I did that to be sure about the distances,
when using ramps, try to use the most wide ramps you can use, or even use normal terrain like Terenas Stand, try that the ramps and paths to be wide, avoid chokepoints and bottlenecks where heroes can be trapped and killed. The 6 ramps of the tavern area seems small.
see you are all doing well, nice. Didn't get notifications for a long time, I thought something went wrong with the contest //
Here is a quick layout wip, nothing too fancy and I don't know if I'll actually go with this one - dropping it here just in case.
@Blood Raven your map does seem really big. Maybe you could create more paths without breaking the theme? I don't know. (you could use invisible platforms and debris doodads to simulate bridges, something along those lines)
Great, Lordaeron summer tile-set appeared!!, looks nice.
Hello! As promised here is the wip image:
https://i.imgur.com/E0dnz0b.jpg
Would love any feedback
96X96
Seems everything is all well placed for a melee map, I can´t say more until look inside the editor, creeps types and loot.
@RFWH
No, it does not necessarily require World Editor Extended, but it of course helps (marginally).
Sky sphere is a model - generally Warcraft III uses a sky box model to display the sky. The default sky box is just two planes that rotate around the camera, so that wherever you look, you see the two planes with the texture of the sky (which ever you use). Sky sphere is a sphere-shaped model that like the planes, display the sky texture - to use it you simply import a sky sphere model into your map and "replace" the default sky box model. As the model changes from two (vertical) planes into a sphere, it is able to also cover the top and bottom of the camera-view.
Alpha tile is simply a terrain art texture that is completely transparent (Alpha = 0). The ground that uses the alpha tile texture is also transparent. Usually you would see black void, as the standard sky box model does not cover the "below of the ground" but using sky sphere allows you to display sky beneath the ground, thus giving the immersion that the ground is up in the sky.
EDIT:
Finished my entry. Here is link to the resource:
(2) Torn World
I saw the layout in the WIP before and I think is very clever, glad that I can see the details inside the map now.
edit:
outland creeps were a surprise, and a lot of doodads, nice work,