The Fort increases the popping power of all nearby towers by 1.
~ Official Description
Monkey Fort is the first upgrade on path 2 for the Monkey Village. It increases the popping power of each tower in its radius by 1, making it extremely useful on any tower with low pierce. It is highly recommended for towers like the Super Monkey or the Dartling Gun, as it allows to pop 2 bloons instead of 1, making it a lot more effective.
This is the only upgrade for the Monkey Village that does not give it a radio tower.
Even if the Monkey Fort is combined with a tower that cause effects in the impact like the Glue Splatter or the Bomb Tower, Mortar Tower, it still increases the popping power by 1. (In Glue Splatter's case, +1 splats) However, since those tower themselves already have very high popping power, they're buffed very little.
Spike Factory within Monkey Fort's radius creates one extra spike that pops one more bloon.
The Monkey Fort doesn't affect these towers and agents:
Monkey Fort turns a Super Monkey into half of a 1/0 Super Monkey that only shoots one projectile (even though it doesn't pop frozen bloons), making this upgrade worthwhile if the player's Super Monkey(s) are in radius of it.
In Version 4.3.1 in Bloons TD Battles Mobile, the Monkey Fort has been nerfed so that it no longer duplicates Glaives from Glaive Riccochet, and in version 4.9 it no longer duplicates Ninja's Flash Bombs, Buccaneer's Cannon Ship bombs, and Glue Gunner projectiles.