In every lane, there are 3 towers. They're named by their tiers with tier 1 being the nearest to the enemies and tier 3 the farthest. You must destroy all the lane's towers before you can damage the barracks. The barracks are behind the tier 3 towers. If you destroy the 'Ranged Barracks' or 'Melee Barracks', the corresponding creeps spawn at the lane will be more powerful. If all 6 barracks get destroyed, all the creeps get boosted to Mega Creeps. Mega creeps are so powerful that an entire wave of them is equal to the power of a hero. If you have mega creeps, you will likely win the game.