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Upgrades are a core game mechanic of the Bloons TD series and Bloons Super Monkey. They can be purchased for cash to enhance a monkey's stats, add or replace attacks, unlock Activated Abilities, or combination of effects. Depending on the game, upgrades may come in multiple paths and tiers that restrict how many upgrades can be purchased and in what order; higher-tier upgrades are generally more powerful, but more expensive.
Towers in Bloons TD 6 change their appearances based on their combination of upgrades. The Wizard Monkeys with path 1 upgrades are on the upper left, the ones with path 2 upgrades are down the middle, and the ones with path 3 upgrades are on the upper right. The ones between those are crosspathed and have upgrades on two paths at the same time; there are 64 possible combinations, though their tier 5 upgrades don't change when crosspathed, so the Wizard Monkey only has 52 unique models. The Wizard Monkey's Paragon is not included here.
An upgrade path is a grouping of upgrades across multiple tiers. In Bloons TD 6 and Bloons TD Battles 2, towers have three upgrade paths; in Bloons TD 4, towers have only one upgrade path; and in all other Bloons TD games except Bloons Adventure Time TD, towers have two upgrade paths.
An upgrade tier is a ranking of an upgrade within an upgrade path. An upgrade can only be purchased if the previous upgrade on its path has been purchased. Starting in Bloons TD 5, Higher upgrade tiers have additional restrictions.
In Bloons Adventure Time TD, characters have upgrade trees instead of specific upgrade paths and tiers. Upgrades can have multiple prerequisites and mutual exclusion is specific to certain upgrades rather than being based on the path and tier.
A crosspath is a combination of upgrades across two different upgrade paths. This is most prominent in Bloons TD 6 and Bloons TD Battles 2, where some upgrades have unique perks or interactions when combined with upgrades on other paths, though the concept exists in all Bloons TD games with multiple upgrade paths.
Upgrades for towers in the Bloons TD series are usually notated with numbers to represent their upgrade tiers. For example, a Dart Monkey in Bloons TD 6 that is upgraded to tier 5 (Ultra-Juggernaut) on its first path and tier 2 (Very Quick Shots) on its second path is a "5-2-0" Dart Monkey.
Properties of upgrades
Upgrades can have a variety of effects on the subject. In Bloons TD 6 alone, upgrades can:
Upgrades can apply multiple different effects at the same time. For example, Primary Expertise both enhances the buffs the Monkey Village provides to Primary towers and grants it its own attack. Not all upgrades are strictly beneficial; some upgrades have both upsides and downsides (such as Hot Shot, which allows Grape Shot to pop Lead Bloons but prevents them from popping Purple Bloons) or fulfil a radically different role from the previous tier (i.e. Buckshot).
Tier 5 upgrades appear in Bloons TD 6 and Bloons TD Battles 2. Only one of each tier 5 upgrade can exist at the same time, with the exception of Crossbow Master with Master Double Cross in BTD6. In BTD6's Co-op, each player can have one of each tier 5 upgrade, allowing up to four of each tier 5 upgrade to exist at the same time (8 in the case of Crossbow Master).
Paragons are a kind of upgrade introduced to Bloons TD 6 in version 27.0. They require all three tier 5 upgrades of that tower to be on screen, and when the paragon is bought, all of that tower on screen merge into a single unit with exceptionally high stats and damage bonuses against Boss Bloons. The player can sacrifice more towers and Paragon Power Totems when creating them to increase their Degree, which determines their stats. They are extremely expensive and take much more tower XP to unlock than other upgrades. Only one of each Paragon can exist at the same time, even in Co-op. While sometimes referred to as "5-5-5 towers" in the game, they actually belong to their own upgrade tier.
Master Double Cross in BTD6. In BTD6's Co-op, each player can have one of each tier 5 upgrade, allowing up to four of each tier 5 upgrade to exist at the same time (8 in the case of Crossbow Master).
