Back in the days of yore, brave warriors battled with bows and arrows. Which, depending on who you asked, they carried uphill, both ways, to the castle, in the snow, with a mastodon under each arm.

Anyways, during the medieval times and before the onset of serious firepower, the castle was the end all in defense structures. While they could be breached or otherwise neutralized, the castles were impervious to non-siege weapons of the day. Defenders could spend their time focusing on the important tasks inside the walls. But given enough time for unchecked mischief, those pesky attackers could tunnel under the walls or start up some bovine based biological warfare. So the defenders had to make those aggressive bitches recognize, ya heard?

"You want me to do what?"
Even though the attackers weren't waiting outside with gats, there wasn't a damn good reason to stand tall over the parapets and let fly arrows. The attackers could very easily return fire on exposed defenders. The same rule applies to defenders who shot arrows from large, open windows.

A Vertical Smile
The answer is the arrow loop, or more appropriately, the arrow slit. It is a simple slit in the castle walls a few inches wide and several feet high. Imagine that a conical section of the wall is cut out behind this port--say a 90 degree arc that allows a defender to position his body 45 degrees to the left or right of the actual slit, resulting in an enlarged field of fire. The arrow slit allows a defender to loose arrows at attackers while maintaining a significant amount of cover. Unless the attackers came from Sherwood Forest there was not a great chance that an arrow would pass through the small opening and strike the defender.

Trouble with a Lowercase T
As technology advanced and one had to keep up with the Joneses, crossbows became an implement of ranged warfare. While bows could be 5 foot tall English longbows, crossbows did not need a lot of vertical room to be fired. Crossbows needed more horizontal space thanks to the, well, horizontally oriented mini-bow that laid across the top of the weapon. In order to upgrade the arrow loops from Arrow Loops 1.0 to Arrow Loops 3.1, a horizontal slit was cut at about the midsection of the arrow slit. This resulted in a cross shaped pattern, which looks deceptively decorative to me.

Cross Your Eyes and Dot Your t's
Eventually arrow technology advanced to the point where people used guns. The first guns were quite inaccurate and took a shit ton of time to reload but, hey, I think there's something to be said for their psychological impact. Getting tagged by an arrow sucked, but watching someone's chest explode probably takes a little fight out of troops. Arrow loops were adjusted once more to let the defenders make the most out of the ungainly firearms. Arrow Loops 3.1 became Arrow Loops XP with the addition of a circular cutout at the intersection of the lines of the cross. Now all manner of projectiles could be shot safely from within the castle walls onto the attackers.
Before you gripe about this paragraph heading, think real quick. The vertical + horizontal slits of the arrow loop look like a cross, or t. "Dot" the center of it and you have an arrow slit with a gun port. Tada!

The Day the Muzak Died
Unfortunately, castles suffered from a general protection fault in the form of gunpowder. As the size of the guns increased from boomstick to Howitzer, the medieval castles were no longer effective defensive structures. They evolved into the military forts circa 1776.