I decided to continue playing on this version just to understand what the difficulty problem was that people were facing before the updates.
From what I can tell, the Flying Purple slimes make it so that the player has to always be at the frontlines of each slime wave, as any defenses that could attack them (Assuming the player's drone isn't the only means of anti-air defense) get quickly bogged down by the quantity of slimes already attacking from the land.
Due to the large slime contingent on the ground for late game waves, the energy shield becomes a liability, due to it both requiring power to operate not being able to survive more than one or two hits from the land based slimes the closer you place them near the slime spawns or where slime waves combine into large forces (ex. Cliff Bases). The Energy shield Power is an issue, because that makes it even more difficult for the player to be able to power up more than three defenses at the frontlines, which isn't enough to survive the wave without the player present and assisting in repairs/defense.
Putting it simply, for players that don't know what they're doing, there's a fatal flaw in late game defense that boils down to walls/barriers having too little health, combined with a perceived lack of anti-air options outside of the player drone.
So, in that image, I had replaced the Guass cannon with the Ark Field Generator previously, and that defense was able to hold only because I was repairing the Energy Accumulator once I walked to the frontlines (the shield was already destroyed), and then going up to the second level to repair the solar panels as the slimes dropped
by the purple slime damage whatever they touch as they're dropping to the ground.
But, even then, if I wasn't there, those defenses would've been annihilated. Maybe I need to just spam the basic III lasers, but at the point of the game I'm in, I have to be quick in tearing down and rebuilding defenses for experimentation inbetween waves. And that's not to mention how considerably weak the windmills are when under platforms. Though, admittedly, the picture's position is on the top leftmost cliff of the in-game map.
Anyways, that's it for me playing this specific version. Imma move on to the most current one.