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, aka 150 lives. Medium difficulty is well suited to intermediate players. To pass Medium Difficulty, the player must complete Round 50 in BTD2 and BTD3, and in BTD5 the player will have to pass Round 65, making it slightly harder than medium in BTD4. In BTD5, although it is back to round 60 in BTD 6. In BTD 4 onwards round 60 happens to be the first round with a BFB. In BTD4 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.
In Bloons TD 6, the player must pass Round 60, which is the first round with a B.F.B. bloon, in order to win, with the starting cash of 650, and prize of monkey money depending on the track you're playing. Towers are at their normal prices and bloons move at their normal speeds. The rounds start at Round 1, as normal. There are also a few variations based on medium difficulty.
Military Monkeys Only: beat Round 60 using only military towers (Sniper, Monkey Sub, Buccaneer, Heli Pilot, Monkey Ace, Mortar Monkey) and your Hero .