Before I delve into this, I just want to say that this is my own perspective, but I just wanted to cover the game while it is still in early stages as this is when feedback is easiest to work with. 

Also, for what it's worth, I generally have a large array of games under the belt that I played over time, both in legitimate and...less legitimate versions before I was on my own and could afford all the games I wanted. I've played mostly RPGs, Roguelikes and Roguelites, all Endless titles, 4X titles, various live-(and dead...)services, horde slayers recently, like Soulstone Survivors, Spellbook Demonslayers, etc. I could go on and on. I'm also an aspiring game designer and in general I just want to see Amplitude titles thrive and succeed.

Okay, so here goes, as ordered as I could make it:

Balance/Difficulty Curve

Issue:

I can't exactly tell if it's the spikes of damage depending on doors opened, the increased health of the enemies or their numbers, spawner count, but something feels askew here. Maybe it's all of them?

I've hit this situation multiple times while playing - I try to find the door to the next area, to see if I can risk stockpiling some more resources and so on, but I end up with almost all the doors opened because of RNG or a barrier. 

This usually results in the following: Multiple spawners all spread out attacking everything with no efficient way to defend everything early on, as the Generator upgrades most of the time are not unlocked. Lack of matching weapon types also increases the difficulty greatly, but I will cover this later.

It's hard. Very hard, and especially early when you're spread very thin because you can't defend every choke point effectively, as the turrets can't usually handle things themselves very well. I don't know if the third hero will help, because they don't use abilities from what I noticed and you can't micro-manage your heroes as easily as in DoTE.

Damage seems off as well, I've had some damage spikes on my heroes that made my jaw drop with no defensive maluses. I don't know exactly what it was, but it came from both the bugs and the robots.

Suggestion:

Cap the number of spawners that can, well, spawn early on. I've had multiple times 4 to 5 spawners before finding the door to the next section.

2 to 3 max in the first zones and as the game progresses, increase the number to 4 and 5. 

This would make holding choke points easier while allowing easier scaling.

Just slightly lower the high end of the damage curve early on. Almost getting your heroes nearly one-hit in the first or second section feels frustrating. This leads to the next point, below.

Medkits

Issue:

As they are now, I feel like they are too expensive. Mistakes in solo mode are very easy to make as you lack the control you had in DoTE. The Medkits costing 20 is a bit too restrictive. 

Suggestion:

A cost of 15 I say would work better. 10 would be too cheap, I feel, and way too accessible and spammable.

15 makes it so that you still have to weigh between the upgrades for your hero and saving up in case of an emergency if your heroes get damaged.

Or, instead, add a linear curve based on progression. Make the first #N number of medkits cost 10, the next #N number cost 15 and then capping at 20 from there-on.

Generator Costs

Issue:

In the state things are now, you are hard pressed to invest into Industry and Food yet you still end up never having enough because of how the scaling works, almost eliminating Science as an option. This is further emphasized by not getting an upgrade option at the Research Station for either of these.

Suggestion:

Just like the others, perhaps some scaling early on to make the increasing cost cheaper for the generators and normalizing towards early to mid game. Not much else can be done here.

Aggro Range/Area

Issue:

I am trying not to sound salty about this part in particular, because this one annoyed and frustrated me to absolutely no end. Enemies should not attack generators or turrets that didn't draw aggro.

I would often find a generator in an isolated room, off the path from the spawner to the crystal just to be notified during a wave that enemies moved from the main path and is now attacking the generator in the god-forsaken land.

And it happened....EVERY. SINGLE. TIME. Why?! Tower Defense enemies do not behave like this. They don't go off their paths just to mess up with your stuff if they're not in range of each other. Or if there are any they are usually unique/elite enemies specifically designed for that.

Suggestion(This is definitely a request.):

Please, just remove this "feature" entirely. It just adds layers upon layers of frustration and grief while playing. I can't even temper my feelings about this topic.

UI Hero Tracking

Issue:

It is very hard on large displays and just in general to track things like skill CD, mine charges on Blaze, life values and so on. In the heat of the combat, double so, because you just lack control in too many aspects.

Suggestion:

This is tricky, but perhaps add an option for the info from the left side to be presented on the bottom? Would make tracking things easier and center of the screen is where most people are used and expect to see this information.

Weapon Damage Type Dependency/Feel

Issue:

Not having a matching elemental weapon greatly lowers efficiency and overall fun early in the game. They're way too important and skew the balance curve as having to pick them even if you don't want to just because it's the only option. Additionally, the charge weapons I expected to release automatically, but instead you have to do it manually every time.

Suggestion:

Here I have a bit of an odd-ball suggestion. Keep the weapons as they are in their behavior, but add a module or a "prism" you can unlock that allows you to change their element on the go. This would keep the feel for each weapon unique while still keeping them relevant throughout the whole run if you like a particular weapon.

Additionally, add an option to release-fire on full charge on charge weapons. 

Conclusion

Difficulty curve early on is a bit too high in solo play and should be adjusted either focusing on specific aspects or just adjusting values in smaller amounts across the board.

Last, if I haven't mentioned or brought a topic up in the thread it means that there are no changes or adjustments that I feel would be needed and everything is good on that particular end.

I'm really curious as to what others think and their overall experience on these main points I brought up.

I agree with most of this.  I consider Science to be more useful than Food because the hero upgrades are mostly worthless, and using Science to research certain types of turrets can reduce the need for medkits.  The aggro is a little weird, and it's hard to tell which enemies target generators, heroes, turrets, etc.  I'll also add that I don't like it that certain enemies focus on turrets but always stay out of the range of the turret.  If an enemy is attacking a turret, it should do so within the turret's range.  And yes, charge weapons are frustrating and mostly unusable by anyone but the AI.

I do like the idea of allowing you to change your weapon element at will.  I suggested the same idea in my elemental affinity discussion thread and suggested that it may be a good thing to allow for turrets as well.