The Swiss Guard is a mercenary unique to France. Unlike most mercenaries, this unit does not require the "Mercenaries" technology, but can be trained at the castle as soon as Eternal Peace has been declared. Like its equivalent, the Swiss Pikeman, it carries a great attack bonus against cavalry and infantry and also benefits from the long pike.
Available to: France
Trained at: Castle
Requires: Eternal Peace
Cost: 80 Florins
Armor/Pierce Armor: 1/1
Special: +25 vs. cavalry, +12 vs. ships and lancers, +6 vs. infantry.
Swiss Pikemen train incredibly quickly and have better attack and armour than regular French ranged pikes, but they are slow. It doesn't matter if you face melee units, but be aware of archers. So be sure to give him either Skirmishers or Light Cavalry support. If you now also boost them with Close Order Drill, the result is a unit, which will dominate any melee given enough numbers. These guys are expensive, but they are worth their silver.
The downside? You have to pick the Papal Protector PD first and you have to wait until 15th, during which you're likely to use Scots Guards (and priests, and perhaps cavalry) to great effect - if you already developed your economy to grant a constant flow of florins. Then you have to research a tech for 600 food and 800 florins, after which you finally get to train Swiss.
If the Swiss Pikeman was good, this unit, due perhaps entirely to favorable circumstances, is even better.
First hired in the late 15th century by Luis XI, "les Cent Suisses" were among the most frequent and highly skilled troops to join the French forces and soon formed a royal bodyguard of exceptional skill.