As someone with a long-abiding interest in castles, there's a simple mod to the Capitol they could do to make it more riot proof. Castle doors were thick and had bars you could drop across them. For a building like the Capitol they could be appropriately sheathed with engraved brass historical motifs, but strong on the inside. In normal times they would stay open and be just part of the building scenery, with the kind of glass and wood outer doors we saw getting bashed in used to keep the weather out. In emergencies the doorkeepers slam them shut, turn the handle to extend the floor and top pins to lock them in place, and drop the horizontal bar. The photos give an idea what I mean.
It's more than that, though -- in reality, the Metropolitan Police would never let that particular kind of situation develop, since they'd have cut off access the the whole area well before things started to get out of hand, but that the first sign of trouble, the large iron (?) gates to the Palace of Westminster would have been closed.
Then, inside the gates, the police guarding Parliament are armed with machine guns, as are the police guarding the doors to the parliament building itself (which would, of course, be locked too).
I've not been inside Parliament itself for a long time, so I don't know what it's like at the moment, but I'd assume that the police inside the building are similarly armed, or can be very quickly.
Individual terrorists have been able to get inside, or almost inside, the palace precinct, but a mob would never make it that close.