Upgrade

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Upgrades are a core game mechanic of the Bloons TD series.

Terminology

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. 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.

Upgrade

Terminology

Lists of upgrades