The first two maps are Swordbase and Powerhouse; these will feature "party" game types such as Headhunter and Slayer. Alongside those is Overlook, which was actually straight from the campaign, and Boneyard, which Bungie has remained quite tight-lipped about, at the moment. The entire interview can be read here, and I highly suggest anyone with interest on the game read it in its entirety, but I will try to provide you all with some of the most notable highlights:
IGN: Why did you choose these specific maps for inclusion in the beta?
Chris Carney: All four maps were chosen in order to test a specific selection of gametypes and team sizes. Sword Base and Powerhouse cover the Slayer, Headhunter, end of the spectrum, Overlook is perfect for the NT 1* ("Generator Defense") game type, and Boneyard is a great landscape custom built for Invasion. Each map brings unique elements that we want to evaluate and hopefully contain enough variety to be appealing for the duration of the Beta.
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IGN: All four of these maps are human themed, and Reach takes place on a human planet. Does that leave much room for Covenant or Forerunner themed maps?
Chris Carney: All of the maps in the Beta are human themed, but several of the other maps have different themes that I think are really cool. I don't really want to talk about them because it would talk about things that are happening in the single-player story. But, yes, we have come up with a cool palette of environments for ship for multiplayer.
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IGN: In terms of multiplayer map layout, the leap from Halo: Combat Evolved to Halo 2 felt huge due to the removal of ladders and fall damage. How drastic is the leap to Halo: Reach going to feel and how is it shown in the beta?
Chris Carney: Reach still builds on the solid foundation of Halo 3, but we have reevaluated some of the decisions that we made in our three previous multiplayer offerings. While ladders will never surpass the incredible awesomeness of the man cannon, there are some returning elements that should make players reminisce. But will it feel drastic? Not sure, but there are some things that will feel classic (like the return of health packs and moderate fall damage) in addition to all of the new hotness.
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IGN: Do you plan to use data pulled from the Beta to further tweak the level design leading up to the final product?
Chris Carney: We're doing a lot of behind the scenes data collection for the Beta. There's tons of stuff internally that we're pulling from every game; every kill, every death, every bullet fired we're tracking in some way, mainly to help us balance the game towards ship.
Lars Bakken: We will be looking at those kinds of things internally like heat maps, like we did with the Halo 3 beta. We will make changes to maps if we see problems starting to arise. That's a huge part of the beta for us, seeing these maps and game types going out to the public. If there are problems, identify those problems and fix them for ship.
Again, this was only a small tip of the iceburg, so please refer to the link above in order to read the full interview, including even more details on how the maps were chosen, what weapons and features will be available in the beta, and more. Sounds like this is shaping up to be a pretty awesome looking beta. Stay tuned for more details as they emerge!