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 costs $765 in Easy, $900 in Medium. $970 Hard and $1080 on Impoppable. 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.
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.