Question

Which excerpt from the "General Prologue" of The Canterbury Tales tells us that the knight had been part of the Crusades, military expeditions

in which Christians sought to win the Holy Land?
In Latvia raided he, and Russia,
No christened man so oft of his degree.
In far Granada at the slege was he...
And he'd fought for our faith at Tramissene
Three times in lists, and each time slain his foe.
This self-same worthy Knight had been also
At one time with the lord of Palatye..
And always won he sovereign fame for prize.
Though so illustrious, he was very wise
And bore himself as meekly as a mald.

Answer 1

From the lines that are available in the question, the ones that describe more explicitly his participation in the Crusades are:

“This self-same worthy Knight had been also

At one time with the lord of Palatye…”

Here he is described as having served the lord of Palatye. Palatye is the Middle English version of the word Palathia. Palathia was a Christian fief in Asia Minor (modern Turkey). When the Crusaders went to Asia Minor on their way to liberate the Holy Land from Islamic invaders, they established several Christian lordships along the route in order to protect Christian pilgrims from Europe on their way to Jerusalem. Palathia was one of such lordships and it was located in Asia Minor, in what had been part of the Christian Byzantine Empire and had been invaded and conquered by Muslim Turks.