The rock-paper-scissors design of Age of Empires 2 (aka: Age of Kings) is more nuanced than what I described above.
So the meta pre-patch is that XBows are weak to Skirmishers. (10 E. Skirms can often win vs 20 XBows). Knights are weak to Monks (100 gold to convert a 60-food+75Gold unit is effectively a 120-food + 150-gold swing, so the Monk can die afterwards and you still got an effective conversion. Furthermore, Knights are the same speed as all other knights and have a ton of armor+HP, so the converted knight can likely escape).
If the monk survives for a 2nd or 3rd conversion, even worse. The monk already paid for itself after just one conversion.
The "intended" rock-paper-scissors was that Infantry (ie: Longswords) were countered by XBows. But...
You see, its more important to "force" your opponent into making otherwise worthless units. Skirmishers may counter XBows, but they're nearly useless against all other units in the game. So by "forcing" your opponent into a counter-unit, you've more effectively controlled the game.
So Longswords are countered by XBows. But guess what? XBows were the meta-play to begin with, so you just "forced" the opponent into making good units, what he should have been doing from the start!
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That being said, XBows only counter Infantry when micro comes into play: effectively knowing the XBow move-and-attack cooldowns. Its not a thing beginners do. If you just patrol-move into a pile of infantry, the Longswords kinda do okay.
So its only a counter much like how monks "counter" knights, I assume perfectly play on the part of the XBows (who have nearly the same movement speed, so they can perform a "retreating" kiting maneuver against enemy Longswords). If you are unable to micro like this however, its not really a true counter.