A tower is a unit that helps the player fight off Bloons. With the exception of the Banana Tree and the Monkey Village, every tower can attack bloons in BTDX (although both Banana Tree and Monkey Village can attack with the certain upgrades). They are all unique, with increasing power generally accompanied by increasing cost.
When you first start BTDX, each tower will have a total of 5 upgrades available to it, 1, 2, and the first upgrade each of the main three paths, left (3L), middle (3M), and right (3R).
The shorthand name for an upgrade is <tier number><path letter>, where tiers start at 0 for the base tower with no upgrades, and the paths are Left, Middle, Right, or Stacked (sometimes called Alt). So a "3L Ninja Monkey" would have the "Distraction" upgrade.
The stacked upgrade path for the tower will show in place of one of the other paths once unlocked (left, middle, or right)
The "stacked" upgrades (3S, 4S, 5S), also sometimes called "Alt" (3A, 4A, 5A) upgrades, are the 4th path available to towers. You can unlock them by unlocking all upgrades for a particular path (it differs for each tower, for example the Dart Monkey requires that you unlock all the upgrades in its middle path) and completing a bounty using only that tower (via a button that will appear in the lower-left of the tower menu).
Once unlocked, you can buy the stacked upgrades in game by upgrading a tower to tier 2, then attempting to build (or "stack") another of the same tower on top of it. This will show the tier 3 stacked upgrade option in either the left, middle, or right of the upgrade area. Click it to start your tower along the stacked path. Once a tower is at least tier 3, you don't need to try to build over it to continue along the path, and can instead simply buy the 4S and 5S upgrades as normal.
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