So just what was Belial's plan there?

[MongoJerry]

Yeah, see, Maghda was...awful all around. And D1's backstory in the manual at least was actually good - if you don't remember, that's basically what all the books in A1 of D3 recount. The only part I didn't see mentioned was Leoric's paranoid war with Westmarch when he was going insane. Which should have been mentioned since Westmarch gets mentioned in A3.

It is mentioned a couple times in the lore books in the cathedral. The journal of the character who killed Leoric describes his men being sent off to die in a foolhardy war against Westmarch.

Yeah, I don't know what Azmodan is doing at Harrogath Arreat either, I'm guessing his job is to be a distraction for whatever Diablo is plotting.

You need to stop escaping through the cinematics. That one is explained directly. Azmodan heard about Belial's defeat and the black soulstone. His plan is to have the hordes of hell pour out into Sanctuary to search for it. Once he has it, his intention is to become the Prime Evil with himself at the head. He pours out of Arreat, because after the explosion of the Worldstone, the barriers separating hell from Sanctuary were weakest there. However, the only reason why our heros know for certain to go there is that Leah has a vision that Azmodan is going to make his move at Mount Arreat. Bastion's Keep is the main choke point separating the hordes of hell from the rest of Sanctuary. Basically, if it falls, the hordes of hell will be able to run rampant throughout the countryside of Sanctuary in their search for the black soulstone.

Azmodan also managed to surpise and overwhelm the defenders at Bastion's Keep. There are several mentions of this. For example, conversations between the character and the templar include mentions that "they could have defended this bridge" but that Azmodan's army struck so quickly, an adequate defence wasn't able to be mounted. Plus there are comments and journals describing about how the keep was considered a less dangerous assignment. The keep had been built as a defense against the Barbarians. With them effectively gone, the keep had not been adequately maintained or manned. (A conversation in town between two npc's I just caught included comments about how they wished they hadn't skipped out on wall repair duties all this time). The armies of hell came so quickly that they didn't even have time to raise the catapults or light the signal fires -- hence the quests to clear off the walls and help raise the catapults.

But, yeah, having Azmodan tell us that his armies have breached the walls below the keep was pretty stupid.