So let me get this right...

[r2000]

Currently in HL2 three dimensions are known to exist.

-The Combine Home Dimension
-Xen
-Earth

Earth exists as one dimension. The resonance cascade allows travel between Xen and Earth. The Xen reach our dimension via the Nihilanth, and we reach Xen via our machinery. Before this, the Combine had also attacked Xen. The Xen races created the Nihlanth to stand strong against the Combine, and ended up being enslaved by the Nihilanth. We destroy the Nihilanth. Xen goes into chaos and the vorts go to Earth to flee. The combine pass through Xen and then takeover Earth. You arrive on the scene, raise all sorts of hell, and the game ends there.

Now, am i more or less right?

 

[Eejit]

I think that the Combine may be spread across more than 1 dimension.

 

[Llama]

Combine do not pass through Xen, they have never been able to get into Xen

 

[ríomhaire]

Currently in HL2 three dimensions are known to exist.

-The Combine Home Dimension
-Xen
-Earth

Earth exists as one dimension. The resonance cascade allows travel between Xen and Earth. The Xen reach our dimension via the Nihilanth, and we reach Xen via our machinery. Before this, the Combine had also attacked Xen. The Xen races created the Nihlanth to stand strong against the Combine, and ended up being enslaved by the Nihilanth. We destroy the Nihilanth. Xen goes into chaos and the vorts go to Earth to flee. The combine pass through Xen and then takeover Earth. You arrive on the scene, raise all sorts of hell, and the game ends there.

Now, am i more or less right?

1: http://www.halflife2.net/forums/showthread.php?t=91660
2: Nihilanth "Their slaves, we are their slaves, we are"

 

[r2000]

I think that the Combine may be spread across more than 1 dimension.

I think they dominate more than 1 dimension, but they're homeworld exists in only 1 dimension.

 

[chimpmunk]

Combine do not pass through Xen, they have never been able to get into Xen

I think this quote, from Lombardi will prove you wrong.
LOMBARDI: We had a glimpse of the larger threat when we were working on Half-Life 1. In other words, we knew that once you cleared out the Nihilanth (end-of-game boss), you were going to discover something worse beyond it. We knew that some immense threat had chased the Nihilanth and its creatures out of their own world and into Xen, from which location they were all too glad to seize the opportunity to continue on to Earth with suppression through the citadels. But the exact nature of the threat was left to be solved in Half-Life 2.

 

[ArchitectX]

When I think of it all it makes my head hurt...

 

[Llama]

I think this quote, from Lombardi will prove you wrong.
LOMBARDI: We had a glimpse of the larger threat when we were working on Half-Life 1. In other words, we knew that once you cleared out the Nihilanth (end-of-game boss), you were going to discover something worse beyond it. We knew that some immense threat had chased the Nihilanth and its creatures out of their own world and into Xen, from which location they were all too glad to seize the opportunity to continue on to Earth with suppression through the citadels. But the exact nature of the threat was left to be solved in Half-Life 2.

Actually, it proves me right. The Xenians fled to Xen and remained there because the Combine could not access it. Check the thread thats based on this quote to read up more.