X-Bow

"The X-Bow shoots mystical bolts with terrifying power. Load it with Elixir and the X-Bow works automagically. You can set it to target ground units at long ranges, or all targets at reduced range."

Summary
The X-Bow was introduced in the Halloween 2012 update along with the level 9 town hall. It is unique in two ways. First it has to be loaded with elixir in order to function. It holds 2000 rounds and costs 50,000 elixir to fill (25 elixir per round). Lastly it can be configured to target only ground units with 14 squares range or both ground and air units at 12 squares range.

Supercell posted a teaser video on their Facebook page prior to the update's release:

X-Bow Teaser Video

Trivia

 * The X-Bow has only 1 level, and cannot be upgraded.
 * It is the most expensive defence based on initial cost.
 * It is the only defence that can change between targeting ground or ground and air.
 * In the upcoming update, the Xbow will be available to upgrade to level 2, and 3.

Defensive
A well-placed and loaded X-Bow will dominate attacks, but the bow is not good to use as a front-line defense. With the longest range of any defense, it is best placed behind all the other defenses, near the Town Hall.

Offensive
The X-Bow has no weakness unless it is set to ground-only mode, determinable by the way it is facing. If it is facing up, then it can attack air units as well. It does 50 damage per second, which is lower than Cannons or Archer Towers, but fires much faster. Treat the X-Bow like a longer-ranged Cannon or Archer Tower, but don't forget that many units are not a problem for this tower.

When attacking you can look to see if the X-Bow is loaded or not. Currently there is no way to tell how many bullets it has left but you can visually tell when it is empty. An X-bow that is empty is completely useless and will not fire.

You can also tell by looking at the angle whether or not it is configured to fire at ground troops or ground and air troops.