There is little to add to the wonderful answer by @Cerberus, but I though it might be useful to add another possible way of translating this sentence into English by rendering the occurrences of quantus as being akin to rhetorical questions by "such":
Ponder, how one night nearly destroyed this empire founded by such efforts, our liberty ...
magna fenestra is fine, but as noted by @Sebastian Koppehel, historia is written history, rather than the historical events themselves which you probably want to refer to.
So I suggest magna ad antiqua fenestra or magna ad antiquitatem fenestra or magna ad vetustatem fenestra.
Whether you say fenestra magna or magna fenestra is up to you – both is absolutely fine in Latin.
If you go with fenestra at all, I recommend using the preposition ad, because there is a precedence from Terence (Heautontimorumenos 3,1,72): quantam fenestram ad nequitiam patefeceris “what a great window to licentiousness you will have opened.”
But that is not ...
Summary: the reason why this sentence seems unusual after translation is only because of the limits of English syntax, not because anything odd in the Latin.
A short form of expression combining two really distinct indirect questions
I do not understand why the commentator read the sentence that way. It is theoretically possible to read the first ...