Luckily the Doctor did not blow up the Byzantium last week: He blew up the gravity globe, which sent gravity spilling everywhere, so when they jumped up they actually jumped down, and now they are upside down, the enemy's gate is down, and they are dancing on the ceiling. Let's talk about it for a long, long time. River gets it, army guys never get anything and also in this case are big shitty fake CGI, Amy for some reason doesn't get it -- "Doctor, what am I looking at? Explain!" which earns her a literal "Oh, come on. Amy, think!" -- and then acts dipshitty some more as he tries to usher them all into a corridor which has its own fake gravity, perpendicular to their current upside down gravity. The whole cast pitches in at this point to explain to Amy what should be immediately apparent, not to mention it doesn't really matter if Amy understands the finer -- or I mean, totally obvious -- qualities of artificial gravity, because those Angels down there are looking more and more like actual Angels all the time.
But really it's because A) We don't trust the audience to understand things, even though we really should and B) We see no problem using Amy as a stand-in for the hypothetical audience's hypothetical retardedness, which is a separate issue but also an obnoxious problem. When you're shoving every single idea you've had into every single episode, and somebody says it's confusing enough that you have to treat one or more characters like they've had a head injury... Well, you get this episode. Which is a rollicking adventure, or rather a serial set of meaningless mini-games, that lasts for an hour and signifies nothing. Which is still more fun than watching River and Donna swan about in the gauzy Lady Fantasy of the Library for an hour, I'll grant you, but is just as much of a letdown coming off the first half. But whereas earlier irritating episodes were impossible to summarize, this one is fairly easy, because nothing actually happens.
The Angels or something closes the team -- River, Doctor, Amy, Octavian, a soldier -- in the immediate corridor, open the outside door, and then come closer, closer, closer. If you still think they're scary, I guess they're scary. The lights start going off and on, the gravity might fail, it's all hugely important. So ramped up is the scale and scope at this point that it actually becomes self-parody: "There's no way to override [whatever]! It's impossible!" River says, and the Doctor says, "How impossible?" and River says, "Two minutes." Which is like everything embarrassing about Moffat happening at once.