One of the things that would be stupid in any other scenario but are actually quite well though-out in this case. The Praguers have almost no proper leadership by this point. They are acting solely out of their own greed, the chain of command more-or-less gone. Now, in any other case a surprise cavalry charge like that would not stand - the praguers would have sentries guarding their perimeter up until the very last moment of the siege and would be capable of preparing a proper defense, be it a makeshift polearm wall or, better yet, a siege within a siege.
The praguers are almost triumphant, but they're also demoralized, leaderless and helpless. Basically no different to a random bunch of bandits.
4:10 You cannot tell me that this particular shot is NOT a reference to "Forth, Eorlingas!" part of the battle of Pelennor Fields scene from The Return of the King. The way camera pans up to show the cavalry ready to charge is near identical.