Track Difficulty

Please see Difficulty if you are looking for game difficulty.

Track Difficulty is how hard or challenging a track is, according to the developers. This was introduced in BTD4 and was also in BTD4 Expansion. There are 5 track difficulties: Beginner, Intermediate, Advanced, Expert, and Extreme (introduced in BTD5)

Bloons Tower Defense 4
In the computer version of BTD4, there are 9 tracks: 3 beginner, 3 intermediate, 2 advanced, and 1 expert. (asterisk * = available in iPhone game)
 * Beginner
 * Ocean Road*
 * Sweet Tooth*
 * Fireworks Track*
 * Intermediate
 * Tap Track
 * Pool Table*
 * World Tour*
 * Advanced
 * DNA Test*
 * River Bed*
 * Expert
 * Ant Hill*

Bloons Tower Defense 4 Expansion
In BTD4 Expansion, there is 1 track for each track difficulty. Although, the Premium Tracks from BTD4 are included in BTD4 Expansion, adding to the track total (see below for the premium tracks). Here are the 4 "base" tracks (asterisk * = available in iPhone game):
 * Beginner
 * Beach Track
 * Intermediate
 * Sewer Track
 * Advanced
 * Monkey Temple*
 * Expert
 * Triangle of Insanity

Premium Tracks (BTD4)
In the Premium Tracks of BTD4, there are 7 tracks: 1 beginner, 2 intermediate, 2 advanced, and 2 expert. There is also a Track Pack that includes all the tracks of the premium tracks at a discounted price. (asterisk * = available in iPhone game)
 * Beginner
 * Daisy Chain*
 * Intermediate
 * Blue Laser*
 * Clifftop Track
 * Advanced
 * Grasslands Track
 * Inter-Dimensional Track
 * Expert
 * High-Tech Track
 * Rail Track*

Bloons Tower Defense 5
In BTD5, there are 14 tracks:
 * Beginner
 * Monkey Lane
 * Park Path
 * The Rink (added on Feb 2, 2012)
 * Intermediate
 * Bloon Circles
 * Archipelago
 * Snake River (added on Apr 4, 2012)
 * Dockside (added on Jul 19, 2012)
 * Advanced
 * Mount Magma
 * Switch
 * Lightning Scar (added on May 10, 2012)
 * Expert
 * Clock (unlocked at Rank 28)
 * Castle (unlocked at Rank 34)
 * Death Valley (unlocked at Rank 40, added on Aug 3, 2012)
 * Extreme
 * Main Street (unlocked at Rank 50, added on May 10, 2012)
 * Bloontonium Lab (unlocked at Rank 60, added on Apr 4, 2012)