IGN AU just came out of interviews with Lars Bakken (Multiplayer Designer) and Frank O'Connor (Writing Lead) at Bungie. Straight from Lars, here's the description of the newest map, 'Narrows':
"It's a Forerunner structure. It's embedded in this cliff wall and it's a bridge. Imagine the - if you're familiar with Halo 1 - imagine the bridge from Assault on the Control Room, expanded, and made for multiplayer, so it's got multiple levels and each side has kind of a dug in section, and it's mirrored so on one side of each of the bases there's actually a Man Cannon, and if both sides take it at the same time they'll actually meet in mid-air, which is pretty cool."