As the description says, the Goblin Barrel can be launched anywhere, allowing players to attack or finish off buildings they cannot reach. However, it is easily countered with spells and area damage troops.
If left ignored, the Goblin Barrel will do significantly more damage to a Crown Tower than an equivalent level Rocket for less Elixir. At tournament standard, the Goblins will stab a Crown Tower seven times, dealing 693 damage. If the Goblin Barrel is placed at the centre of a Princess Tower, it will cause the spawned Goblins to split apart on all sides of the tower, making them harder for splash units to kill.
If you observe closely to a Goblin Barrel placed in the center of your Crown Tower, the Goblins actually are clumped together before they spread outwards. This is why splash units will sometimes hit all three Goblins before they are separated.
The Goblins from the Goblin Barrel have high damage per second, but relatively low hitpoints. If it is being tanked by a high hitpoint troop that distracts the Crown Tower and any defending units, the Goblin Barrel can deal extremely high damage in a relatively short amount of time.
The above strategy also works vice versa. If you have a troop that deals high damage but has moderate or even low health, deploy the Goblin Barrel first and then drop the troop. This strategy is effective with Hog Riders or even Mini P.E.K.K.A.s. The Goblin Barrel will draw enemy fire while the other troop easily destroys the opposing Crown Tower.
As Goblins have relatively low hitpoints, they can be eliminated relatively quickly by Crown Towers. When using a Goblin Barrel to attack a King's Tower, either distract or destroy the Towers to prevent your Goblins from being destroyed before any noticeable damage can be done.
The Goblin Barrel can be used to force the opponent to use their spells, allowing the player to swarm their opponent with cards such as the Skeleton Army and Minion Horde. However, this strategy can backfire if the opponent has some splash damage troops or a Mirror. If you have a Mirror, then you can send a higher level Goblin Barrel at the tower again, forcing them to react with less effective counters.
When countering the Goblin Barrel, you must not forget its potential damage output. However, you must not over-react and play a high cost card such as a Bowler unless you are confident that you can turn said card into a counter-push, since the Goblin Barrel only costs 3 Elixir.
Spells like Rocket and Lightning are impractical to use against a Goblin Barrel since it is both a negative elixir trade and may not hit all of the Goblins. However if you are 1-0 in a match and your tower is at a low health you can use them to clear out a barrel.
Some spells such as Zap and Poison won't instantly kill the Goblins, so you will take some damage. Other spells however do kill the goblin barrel. However, the opponent may launch the Barrel past, to the side or just in front of your Tower, meaning you may miss with your spells. Stay vigilant and pay close attention to the Barrel's shadow and trajectory when mid-air and react accordingly.
Zap will not kill the Goblins at tournament standard, so you will receive some damage. However, a Zap 1 level higher than the Barrel will instantly kill them.
The Log is the best counter to the Goblin Barrel. A level 9 The Log will kill Goblins at all levels except 13 and 14 (from a max Mirror), meaning that The Log is a reliable counter in tournament standards and on ladder.
Arrows are a very reliable counter to the Goblin Barrel because of its large area of effect, and level 9 Arrows can all kill Goblins from the barrel except when they are level 13 and 14. In the rare case that the Goblins do not die to the Arrows, it is easy to level them up to compensate.
A Tornado placed 2 tiles to the left/right (depending on where the Goblin Barrel is flying towards) of the middle of the King's Tower, 1 second before the Goblin Barrel lands will not only prevent all damage from the Goblins, it will also activate the King's Tower.
A Fireball will get the job done no matter what level the Goblins are, but it is a negative Elixir trade. If nothing else can be played to kill a Goblin Barrel, or it needs to be destroyed urgently, use it anyway because the Goblins' damage potential is too high to ignore.
If you have no spells, then swarms such as the Skeleton Army and Goblin Gang will be able to negate the damage. If played behind your Princess Tower, both will cover all positions except the front tiles. Fire Spirits are another great counter, although they need to be played earlier then the previous two for the best results. However, be wary of spells such as Zap as they can easily clear the area.
If you place the Valkyrie one tile up from the bottom-outer corner of your Princess Tower, she can catch 2 Goblins in her swing and only let the Goblins get 1 stab on your tower. Also, you can place the Valkyrie ahead of time and get all three Goblins because of her instant first attack speed.
Good outcomes can be achieved with troops that can 1-shot Goblins; such as the Lumberjack. Of course, these counters are very situational and depends on what else you have to counter Goblin Barrels because your opponent can throw their Goblin Barrels anywhere they please, messing up these counters.
Ranged units aren't affected by placement since as long as all three Goblins are in range, the result will still be the same. Archers, Musketeer, Wizard and other ranged units prevent all but two stabs on your tower, and a Dart Goblin only lets three stabs on your tower.
Placing a Bowler behind your Crown Tower will take out all three Goblins with the right timing. Just like The Log and Arrows, a tournament standard Bowler can kill all but level 13 and 14 Goblins. Release him once the Goblin Barrel's shadow passes the enemy's Princess Tower. If the opponent tries to mix up their placement, the Bowler will still crush the Goblins anyway. He can then be used to build a counter-push.
This can also be done with Hunter but does not work if they mix up their placement.
Due to a similar attack mechanic and counter-push potential, the Executioner can substitute the Bowler. He can destroy two Goblins in one throw, and the Barrel will get two stabs on your Crown Tower.
There is a precise technique for the Electro Wizard's spawn damage to splash all three Goblins. Deploy your Electro Wizard right when the Goblin Barrel's shadow crosses the river and the Goblins will get a maximum of one stab on your Tower. For this to work, the Electro Wizard must be placed directly below a Princess Tower.
This can also be done with the Mega Knight, and will instantly kill the Goblins, although he costs more and you may need him for other situations.
The Goblin Barrel was added with Clash Royale's soft launch on 4/1/16.
On 11/1/16, a Balance Update decreased the spawned Goblins level by 1. Prior to the balance, a max level Goblin Barrel could spawn level 14 Goblins, which was 2 levels higher than the max level at the time.
On 19/2/16, a Balance Update decreased the Goblin Barrel's Crown Tower impact damage by 20%.
On 21/6/16, a Balance Update decreased the Goblin spawn time to 1 sec (from 1.2 sec).
On 4/7/16, the Tournaments Update decreased the Goblin Barrel's Elixir cost to 3 (from 4), increased its Goblin spawn time to 1.2 sec (from 1 sec) and removed its impact damage.
Before the May 2016 Update, the Goblin Barrel was able to summon level 13 Goblins while the maximum level for the Goblins card was 12. The Witch was able to do the same with her Skeletons.