If Constantinople had fallen in 1100, the Turkish army would have descended upon Europe four hundred years earlier, and the flourishing late medieval culture that was beginning to emerge at that time might never have developed:
The problem was, though, that the Fourth Crusade (1204) sacked Constantinople. In doing so, they fatally weakened the Byzantine Empire - which was Europe's key defence against Muslim expansion.
That the Crusades ever kicked off at all was because of an appeal for help by the Byzantine Emperor, Alexios I Komnenos. In 1095, he asked for assistance from Western Europe to resist incursions into his territory by the Seljuk Turks. What the emperor envisaged was an addition of another force of mercenary soldiers to reinforce his army - an army which already contained a large number of mercenaries. (Those mercenaries included Muslim troops - the matter was a lot more involved than just "Good guys wearing white hats, bad guys wearing black hats").
However, what the emperor wanted and what he ended up getting were two quite different things! While the First Crusade did take a little of the heat off the Byzantine Empire, in the longer term the Crusades weakened it to the point where it no longer served as an effective bulwark against Turkish invasion.
The real unsung heroes in defending Western Civilisation would have to be the Poles. Twice (in 1683 and again in 1920) they saved Western Europe from disaster.
In 1683, Polish forces under Jan Sobieski lifted the Siege of Vienna and inflicted a decisive defeat on the Ottoman Turks, thus saving Western Europe from invasion. Then again in 1920, when the seemingly unstoppable Red Army was poised at the River Vistula and looking certain to burst into Western Europe, it was routed by the Polish forces under Jozef Pilsudski. Were those Soviet armies not defeated at the Vistula, there would have been very little to stop them from sweeping through to Germany and beyond. The "Iron Curtain" would have been in place a generation earlier than it was - and would have bordered right on the English Channel!
(The Battle of the Vistula is well described in Dennis Wheatley's Red Eagles)