i went on a smaller version of it at the Glasgow fair once at the SECC.
Was really cool sets, had 2 marines with us, a woman who didnt look very tough and a guy who looked hard as nails. They looked just like the marines from the movie, except they had british accents
As stated above, it starts out with a little tour, then the lights go out, alrams sound etc and the marine says to stay frosty
As you make your way around the base you came to a kind of conference room where the 2 marines locked the door behind you and went for a little wander to see where everyone was. Only 1 marine came back, running at us and telling us all to move! He left us again and said to one of the visitors at the back of the room to watch that door and if an alien came in, it was his responsibility to stop it lol.
Mate of mine almost had a heart attack when we stopped in a corridor and he felt something on his shoulder, looked up and it was the aliens hand
The alien was going to take another mate of mine away, but he shoved a little 12 yr old girl into its hands instead and bolted like the rest of us
It was loadsa fun and really, really intense. Hated going near dark corridors for a week or so afterwards
I know someone who was banned from all Alien war attractions, the alien was coming for him so he headbutted it