"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."
The X-Bow is a rapid-firing turret that is unique in two ways. First, it must be loaded with Elixir in order to function. Second, it can be configured to target ground units only (at a 14-tile range) or both ground and air units (at an 11-tile range).
Supercell posted a teaser video on their Facebook page prior to the X-Bow's release in October 2012:
A well-placed and loaded X-Bow will dominate attacks, but to be most effective it should not be used as a front-line defense. With the longest range of any defense, it is best placed behind all other defenses, typically in the center of a base (near either storages or the Town Hall).
Remember to refill your X-Bow with Elixir! An unloaded X-Bow is not only a waste of space, it is a good indicator that your base may be neglected and ripe for attack.
The X-Bow has essentially no weaknesses unless it is set to ground-only mode, determinable by the way it is angled. If it is horizontal to the ground, it can target ground troops only. If it is tilted up, then it can attack air units as well.
In order to attack an X-Bow, treat it like a longer-ranged (and much stronger) Cannon or Archer Tower, but don't forget that many units are not a problem for this tower. Distract it with high-health units (Giants, P.E.K.K.As, etc.) and destroy it with a large group of attackers such as Archers or Barbarians.
Important strategy tip!
Attacking players can see if the defender's X-Bow is loaded or not. Currently there is no way to tell exactly how much Elixir it has left, but a player can visually tell when it is empty. When it is loaded, you will see a small box containing what appears to be small Elixir bolts. An unloaded X-Bow will be lacking the clip, and the bowstring will be missing as well (see image gallery below).
An X-Bow that is empty is completely useless and will not fire. It still must be destroyed to achieve 100% destruction, and troops that preferentially attack defenses (Giants, Balloons, Hog Riders, and Golems) will still target an unloaded X-Bow if it is closer to them than an active defense.
X-Bows undergo significant visual changes at levels 3 and 4.
At level 1, the X-Bow consists of a huge crossbow-like structure mounted on a base made of light-colored wood.
At level 2, the X-Bow's base gains gilded legs and rotational mount.
At level 3, the X-Bow base becomes solid black with large black spikes on each corner.
At level 4, the X-Bow base become golden and the large spikes on each corner turn red, like they are glowing with heat.
It is also currently one of only two defenses (the other being the Inferno Tower) that needs to be periodically reloaded.
In ground-only mode, the X-Bow has the longest range of any defensive building in the game at 14 tiles. When the X-Bow is in ground and air mode, the Mortar has an identical 11-tile range (although the X-Bow can attack both air and ground units at this range unlike the Mortar).