The answer to this question is relative - Europe was protected a bit, but was it protected more than in other times? was this protection effective?
On the one hand: Europe was protected quite well. Feudalism was a time of when there was a full group of people -the knights- who devoted themselves completely to the defense of their lands.
On the other hand, it was not completely effective. For example, it did not stop the Mongols, who invaded Eastern Europe (up until Poland and Hungary) in 13 century.
- So Europe was not completely protected, but it might be possible that without knights, Europe would have been even more vulnerable.