There will be more cover as mentioned above like crates, dislodged snow and ice mounds and that sort of thing.

The idea with the 3 routes is to give 3 levels of difficulty up, but each with pros and cons. The main ramp up would be easiest but is most likely to be defended. The first ladder route that goes all the way up is the most direct route, but if defended is potentially fatal, and the second ladder route has a better chance of an attack succeeding because the ladder goes halfway up and then it is a small ramp the rest of the way up and a series of small storage rooms that lead to the control room. The idea here is that it will take the defenders time to make it to the ladder giving the attackers a chance to get up there first and establish a defence of that position. I'm also thinking of putting in blast doors to the two storage room that have to be destroyed with det packs to get through. This would slow the defenders down further as far as getting to the ladder is concerned as they'll have to open these doors first. Also with defenders usually concentrating on the main ramp and the full ladder route, you can usually sneak around the back of them using the 3rd route. It will be a rare situation where the defending side has enough men to fully defend all 3 points, and if they mine the route that'll take out the first guy, but the next guy will make it through.

I see the point now.  The quick access that has the two ladders that go all the way up is good but there is a room inbetween them.  The rebels defnding can see all the way down. 
How about the first ladder up brings you into the room but the second ladder after that is on the other side of the room so the imps have a place to regroup without being bombarded by frags and the rebels cant see them until they are in the room.  hopw i explained it right!!!