On Easy, towers cost less than normal, the bloons move a little slower, and you have 200 lives. Winning in Easy rewards you a bronze medal for that track.
~ Description in BTD5 when mouse scrolls over the Easy Difficulty icon
Easy Difficulty is one of the three difficulties in the Bloons Tower Defense 2-5 ever since Bloons TD 2. On Easy, towers are cheapest compared to Medium and Hard, and the player has, traditionally, double the lives as compared to Hard, with 200 lives, medium with 150 lives and hard with 100 lives (with 100 lives on Easy, 75 lives on Medium and 50 lives on Hard in BTD2-3). The player must pass 50 Rounds in Easy Difficulty in all BTD games from BTD2 to BTD5 in order to be awarded a Bronze Medal. Towers cost 85% of the price as in Medium Difficulty. In Easy Difficulty in Bloons TD 5, bloons also move slightly slower than bloons in Medium Difficulty. Many players use the Easy Difficulty for the short time it takes to get to Freeplay Mode, or for beginners starting the game. When in Freeplay Mode for Bloons TD 5, playing on easy means that, even though you get XP slower, you would get it much easier than in freeplay for medium or hard.