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PHYSICS
Lombardi: The physics festered their way into the game through the results of our play testing (which we do for months before any QA testing begins). Ravenholm, the original home of the 'physics part of the game' occurred a bit later in earlier versions, and it was the only place you had the gravity gun. But, as more and more testers told us this was gameplay they enjoyed (and we could start eliminating fears of being compared to bad experiments with physics in games), the closer Ravenholm moved to the start.
While this doesn't say whether or not the Combine knew about Xen pre-Half-Life (It could be taken as either the Combine chased the Nihilanth/Vortigaunts to Xen and then lost track of them, or that they Combine chased the Nihilanth/Vortigaunts into Xen and beat them there) it at least confirms that the Combine knew about the Nihilanth's race long before Half-Life.STORY
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.