If left ignored, the Goblin Barrel will do significantly more damage on a tower than an equivalent level Rocket.
The Goblins from the Goblin Barrel have high damage per second, but relatively low hitpoints. If placed accordingly with units that have high hitpoints that will distract 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 low or even moderate 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 quickly be eliminated 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.
Arrows and The Log can be effectively used to directly counter Goblin Barrels.
The Zap is the cheapest counter to the Goblin Barrel, however, if the Zap is the same level as the spawned Goblins, then the Goblins will survive with a sliver of health, allowing them to continue to do damage.
Do not launch the counter spell too early, as early spells have no effect on the Goblins, and Zap and Arrows have a very short cast time.
However, adept players can cast the Goblin Barrel in the far corner of the opposing territory, out of the radius of the defending player's Arrows instinctively cast "on" the Crown Tower. This allows the Goblins to survive.
The Log is arguably more effective than Zap to counter Goblin Barrels, as it can hit troops in front of the Goblin Barrel as well.
If no other spells are readily available, the player can use cheap distracting troops and/or area damaging troops such as Bombers, Fire Spirits or Skeletons to prevent the spawned Goblins from dealing severe damage to their intended target.
As the description says, the Goblin Barrel can be fired anywhere, allowing players to finish off buildings they cannot reach. However, it is easily countered with spells and area damage troops.
The Goblin Barrel can be used to force the opponent to use their Arrows, Zap, or other area damage 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 has a Mirror to mirror the two spells.
The Mirror can be used in affiliation with this card to do extra damage on Crown Towers.
If placed correctly, the Tornado can pull the Goblins from the barrel to the King's Tower which will allow it to activate, and therefore, give a slight upper hand in the game.
If the Goblin Barrel is placed centered on an Arena Tower, it will cause the spawned Goblins to split apart on all sides of the tower, making them harder for splash units to kill.
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 level cap for the Goblins card was 12.
The Witch was able to do the same with the Skeletons.
The Goblins spawned from the barrel are 5 levels higher than the barrel itself.
The Goblin Barrel is one of the three cards that spawns troops anywhere in the Arena, the others being the Miner and the Graveyard.
Arrows and the Goblin Barrel are the only two 'launched' spells which don't push back affected targets.
You can see another goblin's leg sticking out of the barrel on the card image, hinting that there is more than one goblin inside.
The Goblin Barrel has the smallest radius out of all spells with a 1.5 radius.
This song depicts 4 goblins coming from the barrel, instead of 3.