The Classic

The Classic is a biome that consists of the following:

Blocks
Multicolored Track (only surface Circle Guy and Ghosts can spawn and move along, unbreakable without Pick-Man)

Track Outskirts (unbreakable without Pick-Man)

Track Wall (unbreakable without Ghost Whacker, required for Circle Guy and Ghosts to spawn)

Entities
Circle Guy (neutral, goes for Shiny Yellow Dots)

Mega Circle Guy (boss, summoned manually with a mysterious power button, summoned otherwise when a Circle Guy eats an Almighty Yellow Dot)

Ghost (Blue, Red, Orange, Pink; common enemies in the classic)

Scared Ghost (neutral, only spawns when Mega Circle Guy is about to spawn)

Glitch (invulnerable, only encountered during the '256 Curse' debuff, inflicted by Mega Circle Guy)

Shiny Yellow Dot (Background NPC)

Almighty Yellow Dot (Background NPC)

Loot
Pick-Man (OP Pickaxe, 20% drop from Mega Circle Guy

Wakka Staff (15% drop from Mega Circle Guy, summons Circle Guy minion)

Ghost Whacker (100% drop from Mega Circle Guy, OP hammer)

Ghastly Stabber (5% drop from Ghosts, unleashes Pixel Ghosts to attack your foes)