The map's layout is actually pretty nice and looks like it'd allow some some fun 1 vs 1 games.
There are a few "issues" however:

• The way you placed the impassable ground under the terrain objects is wrong. You should know that all terrain objects use real foundations (this means either 1x1, 2x2 or 3x3) and the impassable ground has to be placed exactly on the foundations of those terrain objects (use the marble madness mode to make it easier to see where exactly you placed the impassable ground). I suggest you have a look at one of the existing maps as an example for exactly how to place the impassable ground.
• I'd say there's really an excessive amount of tiberium... The tiberium fields are all rather big and they all have 2 tiberium trees. I'd suggest you make all tiberium fields a bit smaller and give them all only a single tiberium tree.
• The only way for players to get to their opponent is through the blue tiberium field in the center of the map, which also is rather huge by the way. Any infantry walking through it would get heavily injured or die even. Aside from that any harvesters that would dare to harvest the blue tiberium would likely not survive, considering it's the only route the units of your opponent can travel to begin with. I think it's best if you remove the blue tiberium in the center of the map and put 2 (much) smaller fields somewhere fairly close to the middle of the map (but outside of the bottle neck all units have to travel through). The blue rectangle (which represents the visible area of the map) is right on top of the red rectangle at the bottom of the map, which will cause problems. I suggest you always make sure there's always at least 3 cells in between the blue and red lines at the top and at the bottom of the map (to fix this, don't resize the blue rectangle, but instead resize the map while keeping the blue rectangle exactly where it is).
  

First of all, thanks for the extensive and helpful feedback. I adjusted the borders and replaced the impassable grounds. However, I didn't find any maps working correctly in FinalSun that I could use as a reference for underlaying objects with impassable grounds. So I took some guesses - I hope they are correct though.

Inspired by your critique, I massively reduced the starting field to encourage the players to push forward faster. I also deleted the three trees in the middle since they didn't really had a point as they were surrounded by 4 other trees and took important space used to build the center bridgehead in the late game. In any case, I will keep it at 3-5 trees for each player to keep the tanks rolling .

The blue Tiberium, however, stays as it was never intended to be a source of income but as a gameplay element to prevent any kinds of structure-rushes: The enemy will blow the field up (which happens in a nice cascade btw) if he sees an MCV or anything other funny looking objects that try to cross it. on the other hand, you cant just blow it up your self to settle in there before the enemy has defenses at his third expansion, since you would have to shoot your way through the trash left behind by blue tiberium.

All in all, its intended to be a map for people who are new to (or uncomfortable with) the whole pvp idea without making it boring for experienced Players.

Tell me if there are any problems with the impassable grounds so I can correct them. I'd allso be happy about a map that works in FS as a reference

Edit: I didn't really look into opening the Maps.mix yet though D:

The map borders are placed correctly now and the amount of available tiberium for both players is much better now is well
The impassable terrain is still wrong however. You often place more impassable cells under a terrain object than the foundation of the terrain object actually need. You also placed an impassable cell under single trees, even though those really don't need it. You only need to place an impassable cell under a 1x1 terrain object when you don't want infantry to be able to walk through it; this is the case with rocks, while with a single tree it's perfectly normal for an infantry to walk right through it.

When I said you should take a look at one of DTA's existing maps for reference, I meant that you should use XCC Mixer to open Maps.mix and then extract a couple maps so you can view them in FinalSun.

Right, here you go.

That's better, although you're still placing impassable cells under tiberium trees, like I mentioned in the other topic
Another thing that you don't seem to realize is that when you're not in marble-madness mode, the way the impassable cells look in FinalSun, is exactly the same as they'll look when you'll play your map. So this means that when you place a terrain object on top of gray sand (or any other kind of ground that doesn't look like clear ground; roads or pebbles for example), you can't place the impassable ground on top of it or it'll be visually noticeable ingame. To make these cells impassable anyway, you'll have to use the impassable overlay (select it from the "Overlay & Special" menu). Unlike the impassable ground, the impassable overlay is always visible in FinalSun, but it's actually invisible ingame.

I left one of those clear impassables since one wouldn't notice it ingame

Looks like you took care of all of the issues, good job. One optional thing you could still do however, is adding a little more detail to the map. Some areas look a little empty and you could make those areas look a lot more interesting by placing pebbles, rocks, bushes, or trees there. The pebbles are under Terrain/Ground by the way, I don't think you've noticed them yet