I found the idea of being able to die in a clicker type of game quite interesting. I don't know if it's something common in modern clickers since I don't play a lot of them, but I thought it was interesting. Here's some things I noticed while playing:

While I did understand the existence of the retaliation mechanic, I wish there was a way for me to know how much retaliation I've built up, or how much will I build up when buying a stat, since there were times I'd buy an upgrade and then get obliterated some rounds later.

When retaliation did kick in, there were times I didn't understand why and the health would go down so quickly I wasn't able to see the health bar go down; it was almost insta-game over. Maybe letting the player recover a small amount of health per click could help with that?

I did notice that some buildings would take quite a bit longer than others to be taken over, and I assume they're the Bosses you mention in the GDD. If this is the case, having visual difference between your average building and the Gigachad Boss building would be nice.

I assume bosses also have higher amounts of retaliation? There were times I'd go against a new building and my health would be suddenly drained super quickly by the new building.

It'd be also nice to have some visual representation of where that retaliation comes from! Maybe there's even some sort of mechanic you could build around trying to lower or combat the effects of retaliation.

As Nijelous said in the comment above, defence usually ends up yielding better results than health. Maybe you could give some sort of buff or status that can scale with your health. Maybe part of your speed is related to how much health you have, or how much health you have left. Something extra related to the health could help make it more worth to buy.

The art gives me some very old Newgrounds flash game vibes, which was cool. I also really like how the UI was spaced on screen. It gave the building the center of the screen and I found it very intuitive!

Honestly, with more visual feedback over upgrades, retaliation, more possible upgrades in general, I can see this being a really interesting game. This definitely has potential. Nice game!

A neat idle game, simple, done effectively. It would be nice to see the progression of damage you end up taking because some levels it tanks HARD and some it does nothing, also health seems to be a pretty useless stat over defence, which I just spammed to make the tankiest shrooms around. But aside from that, I love the art and the sound design is done well. Gratz on getting a game done!