Zones are a small sector in a World, containing at least 1 of any Monster and sometimes Clickables. Zones are tracked towards the top right of your screen and are used to show your progression through a World.
Regular Zones contain a repeating cycle of set types of Monsters that have low, mid or high HP. Within Zones, Monsters never rise in the base HP value - the exponential rise only occurs after each Zone. This makes progression within a Zone easier over time, as you can rack up Gold to spend on upgrading or improving your Equipment.
Timed Zones are a less common kind of Zone found every 5 Zones; a Zone that includes a 3-minute timer underneath Monster HP. Therefore, every World contains 21 of these Timed Zones (if Boss Zones and the World Boss Zone are counted, but if not then it is 17); it is simply not 20 because Zone 99 is the only exception of the multiple of 5 rule as it is a Timed Zone itself. In such Zones, the player's objective is to kill all Monsters inhabiting the Zone within this certain timeframe - if they don't, they simply setback to the beginning of the Zone.
Boss Zones are Timed Zones found every 25 Zones in a World. They only include 1 Monster, a Boss, having much greater HP than Minibosses would. The Boss can be a larger version of any Monster in the current sector of the World. Prior to the fight, a noticeably longer runway is present. The player is still given 3 minutes in this Zone.
World Boss Zone
The World Boss Zone is also known as Zone 100, containing the final Boss of a World, the World Boss, with only one possible Monster species that depends on the biome type for the current World. Prior to the fight, a significantly longer runway is present. The player is given 5 minutes in this Zone.