There is a strain of gamer that gnashes their teeth at the idea of the Hong-Kong-movie-influenced monk class crashing the medieval European party represented by the three core classes of D&D. I am not one of them. In fact, I have come to relish in an amalgam of the monk class and the monk who endeavors to follow St. Paul's dictum to pray without ceasing (1 Thess. 5:17).
Let me begin with a documentary that changed my mind about how we traditionally see and understand martial arts. It is called Reclaiming the Blade. Ostensibly, it is a documentary that examines role the sword plays in cinema; however, in doing so it reveals that the systematic martial arts that we normally only associate with Asia also existed in Europe. The difference is how these two cultures used gun powder. Whereas the East declined and even outlawed most military applications, the West embraced gun power as a way of making war. These two different approaches resulted in the continued use and lionization of martial hand-to-hand combat in the East and its increasing irrelevance in the West. Consequently, the Western martial arts were almost entirely lost.
Lest we forget, this Western tradition is ancient. The Greeks had the systemized unarmed combat style we know as Pankration. Alexander the Great took this with him as he conquered India. Shaolin Kung Fu is generally credited to an Indian Bhuddist monk often known as Boddhidharma. Thus, there is a link, though tenuous, between Western and Eastern martial arts.
In addition, if one reads the Life of St. Antony, we see the man who many consider to be the exemplar of monastics wrestling demons:
Thus tightening his hold upon himself, Antony departed to the tombs, which happened to be at a distance from the village; and having bid one of his acquaintances to bring him bread at intervals of many days, he entered one of the tombs, and the other having shut the door on him, he remained within alone. And when the enemy could not endure it . . . coming one night with a multitude of demons, he so cut him with stripes that he lay on the ground speechless from the excessive pain. For he affirmed that the torture had been so excessive that no blows inflicted by man could ever have caused him such torment . . . The next day his acquaintance came bringing him the loaves. And having opened the door and seeing him lying on the ground as though dead, he lifted him up and carried him to the church in the village, and laid him upon the ground. And many of his kinsfolk and the villagers sat around Antony as round a corpse. But about midnight he came to himself and arose, and when be saw them all asleep and his comrade alone watching, he motioned with his head for him to approach, and asked him to carry him again to the tombs without waking anybody.
In a fantasy world where sin is personified and monsters roam the wilderness, it is not a big leap to imagine that monastics (who live in and wander through the wilderness) would learn to use what God had given them — their body — as a primary weapon against these foes.
Thus, I happily populate my own Euro-inspired medieval fantasy world with monks and delight as they pray and punch their way through the wilderness making life safe for civilization.