@Acolytus You're making a massive false assumption here; namely, that Desir and his party are the
only additional measure taken behind the scenes.
*While all the background mechanisms are not covered on screen, it is explicitly mentioned that the response time and the strength of the defense force is unusually good. This is an indication that the management of the Tower took additional measures once Desir and co. were commissioned.
*The main BBEG comments on how stupid it is for them to leave the inner corridors unguarded. Now, this could be a result of poorly trained security, with everyone rushing off to the big distraction at the front; it's a tactic used often enough by terrorists and corporate saboteurs of all stripes. But if the management is competent enough to set a defense-in-depth counter-trap for the terrorists, then it would make sense for the inner halls to be un- or lightly-guarded since the remaining response teams would be in hiding, as presenting an image of weakness is very useful for luring your enemy where you want him to be. See Perturabo's Eternal Fortress for an example of the overall mechanics behind that scheme, as well as a Henderson-scale demonstration of all the many tricks you can use in such a defense.
*Desir mentions that between the time he first walks up to the teacher and asks for a commission and the time of the first assault, there's only a little over a 10 hour time gap. While the tower management can certainly take
some additional precautions, 10 hours is not nearly enough resources to prepare a defense against an unknown enemy as you might think.
*A final point to consider is that Desir did not give them all the information he has. If all he says is, "they're going to break down your front gate at X time", then they don't have all the information they need for an effective defense. Desir wouldn't do this if it put the overall mission at risk, but keep in mind he's still operating on a tight time table for preventing the big destruction event; he will manipulate people and circumstances to give himself more options and rewards if he can do so without hurting others.