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I believe citadel will look similar to the fully upgraded Covenant base from Halo Wars also called the Citadel. http://images2.wikia.nocookie.net/halo/images/e/e7/Halo-wars-C ovenant-base.jpg For Heretic, it has already been announced it is a remake of Midship.For Longshore, I am hoping for maybe a remake of Relic, Death Island, or maybe Uplift, but hey thats just my opinion.[Edited on 06.03.2009 7:18 PM PDT]
I think its already confirmed Citadel will be similar to the level Covenant where you have to lower the Sheild Barriers. Here's a link. It says there is a forerunner glyph on the skull so it isn't Covenant related.
Yeah and it also says "It may take place on Forerunner terrain or, given the Covenant's use of Forerunner glyphs, is Covenant in aesthetics. " so it may be forerunner or covenant. Also it wasn't confirmed it was just probable.