Fun for a while
While I had fun with the first 3 columns, the fourth one suffered due to the excessively large map compared to the other sections. (With the cannon the vast majority of units can be sniped before reaching the player) The map size and slow enemy release also make it hard to reach any good scores since the combo meter rests at 0 most of the time.
There seems to be a bug in the big cannon as it has mre power than all other weapons including the ultimate plasma weapon, being able to one or two shot many of the enemies that the plasma would take 6+, leading me to think that there is a bug in the code that allows the bullets to pass through enemies (looks like damage output is counted multiple times)
While I do not mind the AI that endlessly marches toward the player, the difficulty at the end fell due to the lack of variation within each wave as there is never a real mixture of units. (High power collision units just do not have many chances without a large swarm of bullet units to distract the player)