The Templars

The Knights Templar is a holy order which was founded after the first crusade in Jerusalem. Originally they protected pilgrims who travel to Jerusalem, but later they started to expand in other places.

They mostly build holdings in the holy lands, but they can also build in other places, most of the time near muslim borders and they only build castles and temples. After you win the crusade or a holy war and take the land, they might ask you to build their castles in the land. If you accept it, you'll gain piety and pope's opinion of you will be a bit more positive. (Even there are no more holding slots left, if accepted, they will create new slots and build there.)

If you are playing as a Catholic ruler, you can hire them with piety, and when you're defending against infidels, you won't have any maintenance cost at all. But should you remember that they will only fight against heretics and infidels (and rebels as well).

There are four other Catholic holy orders called Teutonic Knights, Knights Hospitaller, Knights of Caltrava and Knights of Santiago.

Trivia

 * The official name of the Knights Templar is The Poor Fellow-Soldiers of Christ and of the Temple of Solomon.
 * Although known for their heavy participation in the Crusades, they are also known for being one of the wealthiest Western Christian military orders, and having a prominent foothold in Christian finance. Non-combatant members of the order managed a large economic infrastructure across Christendom, and improved and innovating financial systems that were early forms of banking.
 * Legend has it that the Knights Templar discovered both the Holy Grail and the Ark of the Covenant after the first Christian capture of Jerusalem, and later smuggled it out of the Holy Land.
 * After the failure of the Crusades, favor for the Order fell. In 1307, after brooding mistrust, King Philip IV of France ordered many of the Templars in France to be arrested, tortured, and burned at the stake for heresy. In 1312, under pressure from other monarchs, Pope Clement V officialy disbanded the Order.