It appears to be a crossbow with wooden and rock features, blue-grey plastic bottom parts and a shining steel bolt which is its ammo. A rope pulls it behind with each hastened shot.
An X-Bow is an excellent defensive building due to its extremely long range and ability to take down large numbers of small troops such as Goblins, Spear Goblins, and Skeletons.
When defending, you can place the X-Bow behind your Crown Towers as it will be well protected but still able to fire upon attacking enemy troops far away.
It can also be used offensively due to its range, being able to out-range the Crown Towers, making for some extremely humorous battles (11.5 vs. 8.5 tile range).
Note that the Mortar can do the same thing to Crown Towers, but it deals significantly lower damage per second, has a slower fire rate, has a slightly shorter range (11 tiles), and does Area Damage.
The Rocket is the easiest and most effective counter to the X-Bow, due to the fact that the Rocket is slow and that the X-Bow is stationary. Its high damage and its capability to be placed anywhere can easily take out a large portion of the X-Bow's hitpoints.
It can also destroy troops or buildings guarding the X-Bow.
In addition, you will have ample time to prepare for the X-Bow, as it takes 4 seconds to deploy. Overpower it with building-targeting troops, like Giants and Hog Riders, and back them up with Archers or Spear Goblins. Together, they will take down the X-Bow quickly, before it can target your Crown Tower. Even if you are not fast enough, the troops can act like a tank for your crown tower.
An X-Bow backed up with some support troops or buildings, such as Spear Goblins or a Tesla, can effectively destroy a Crown Tower. Place the X-Bow behind or near such cards so that it will be protected while it destroys the opposing Crown Tower.
High hitpoint troops, such as Giants, can distract enemy troops/buildings, allowing an X-Bow to safely target and attack the opposing Crown Tower or enemy troops while the "tank" takes all the damage.
If your opponent's X-Bow has locked onto your Crown Tower, and if you have already placed a distracting card, you can use Zap or Ice Spirit to make the X-Bow re-target the other card instead after the stun ends.
When dealing with an offensively placed X-Bow, it's quite effective to deploy a Musketeer behind the river/chasm while the X-Bow is deploying, taking out a large portion of its hitpoints before it's ready to fire.
However, beware of opposing defensive buildings/troops next to the X-Bow, as they will take down the Musketeer quickly if she is in a bad position.
To protect the Musketeer, it is advised to place a tank such as an Ice Golem in front of the Musketeer to absorb any incoming fire from enemy troops, buildings, or the X-Bow itself.
Casting a Rage on an X-Bow will increase its hit speed, rapidly inflicting damage to the targeted unit/Crown Tower, and will boost the player's troops defending the X-Bow. However, the Rage will also reduce the X-Bow's lifetime.
Note that this costs 8 Elixir, which is very risky to play!
An X-Bow can destroy a Crown Tower single-handedly if it's ignored by the opposing player.
It's wise to deploy it alongside a card with a good amount of hitpoints (the Knight) or a swarm card (Skeleton Army), as they can slow down enemy troops sent to destroy the X-Bow.
An X-Bow may need protection from a Bomb Tower or a Valkyrie, as it can be sometimes overwhelmed by the Skeleton Army.
Mirroring an X-Bow can be risky and costs a very heavy 13 Elixir, but you can deal massive damage to the Arena Tower.
The X-Bow was released with Clash Royale's soft launch on 4/1/16.
On 2/2/16, the February 2016 Update decreased the X-Bow's hitpoints by 15% and now only targets ground (from air & ground).