Wikipedia claims that they formed the majority of Mamluk ranks in the late 14th Century. There is also an unsourced paragraph on this page that states:
The Ottoman Empire, which ruled most of the area south of Russia, considered the Adyghe warriors to be courageous and well-experienced. It encouraged them to settle in various near-border settlements of the Ottoman Empire in order to strengthen the empire's borders.
Were there any key events or battles that led to Circassians gaining this reputation?
^Sorry ^if ^the ^question ^is ^a ^bit ^vague ^... ^I'm ^not ^entirely ^sure ^how ^to ^phrase ^it!
The Circassians did indeed have a reputation as fierce warriors. As with many peoples who gain this reputation, it was forged through centuries of tribal style warfare in the rough terrain of the Caucasus. Being muslims at the edge of the muslim world, they fought in raids and counter-raids for hundreds of years.
The Russians needed 100 years to conquer the Circassians and in the end many of the Circassians refused to swear an oath of allegiance to the Czar and were either deported or moved voluntarily, mostly to the Ottoman Empire, but also to Persia. Such was their reputation that the Ottomans offered any that wanted it refuge. About 500 000 Circassians left the Caucasus and ended up in mostly in Anatolia and Syria.