Select the track you want then the difficulty. In the game buy your towers from the side bar on the right, then place them around the track to maximize their effectiveness. You will unlock new tower types with every rank you go up.
BTD4 features improved graphics, loads of new and original tower types and tons of upgrades for each tower type. Heaps of tracks, save games, career mode, sandbox mode and apopalypse mode. Bloons Tower Defense 4 will give you almost unlimited replay value. Also included is optional MochiCoins premium content featuring lots of extra tracks and special booster upgrades like double boomerangs, exploding darts, and double cash!