The Machine Gun is a tower that shoots multiple guns and is effective at destroying groups of units, especially infantry.
The Machine Gun starts out as the Redoubt at Levels 1 & 2 and becomes the Machine Gun at Level 3 and up.
Heavy Cavalry can destroy the Redoubt effectively as the Redoubt struggles against cavalry.
As of update v2.0.x, Machine Guns can attack airplanes.
Visuals
In the level 4 Machine Gun tower the man in the tower is using a U.S-made Browning 30 cal. Machine gun (or the M1917 Browning Machine gun as it is normally called)
Defensive strategies
The best way to destroy troops is to put it next to an Anti-Tank Gun, much like the Mortar, so the infantry will be destroyed and the Heavy Cavalry will too. The Anti-Tank Gun will take out the cavalry, while the Redoubt takes out the infantry that the Anti-Tank Gun struggles against.