On Medium, the towers cost the normal amount, and you have 150 lives. Winning the Medium difficulty awards you a silver medal for that track
~ Description in BTD5 when mouse scrolls over the Medium Difficulty icon
Medium Difficulty is a Difficulty in the Bloons Tower Defense 2-5, making its debut in Bloons TD 2. Towers are fairly priced in Medium. Traditionally, Medium Difficulty gives the player 75% as many lives as Easy Difficulty, and 150% as many lives as Hard Difficulty. Medium difficulty is well suited to intermediate players. To pass Medium Difficulty, the player must complete Round 50 in BTD2 and BTD3, Round 60 in BTD4, and in BTD5 the player will have to pass Round 65, making it slightly harder than medium in BTD4. In BTD5, round 60 happens to be the first round with a BFB. Some players think of it as the boss of medium difficulty. In BTD4, Easy and Hard do not have a single "boss" for their final level, but rather a very tough round with a combination of tough bloons like the BFB, MOAB, etc. However, in BTD5, Medium does not have a Boss Bloon, but Hard does (Z.O.M.G.) While Easy has no single boss, it is sometimes considered to be the M.O.A.B., as it is the last bloon on round 50 (the same can be said for Medium with the BFB). Also all the tower prices are fairly priced.