I like the merging mechanic being hands-off. It doesn't encourage the player to micro-manage things, and it's interesting to look at while it happens. The only thing I'd suggest for this game is more interesting sprites, but this is what I'd consider the platonic ideal for this genre. Good work all around.
Being able to stay on certain floors would be good. Right now floor 13 is basically the only one that gives me a noteworthy amount of experience, but floor 14 destroys me instantly.
The rotation system isn't great. In theory, it means that when you're active, you can distribute damage however you like. In practice, it means that your weakest unit just dies when the enemy spawns on the wrong side of your train.
I like this little game. It's got a lot of reasons to keep a run going for as long as you can manage it, and failure feels like an "oops" rather than a necessity. If I could give one criticism, I'd say make the scroll bars brighter. I spent a good chunk of time going "Man, I wish I could increase gold drops" before realizing whoops, the Rebirth bonuses have a scroll bar.
The lootbox mechanic for tower unlocks is unnecessary busywork at best and frustratingly blatant cash grab at worst. Shame on you, you're better than this.
 "Yet another Merge Game"){kind=icon}
 "Theory of Magic"){kind=icon}
 "Incremental Adventures"){kind=icon}
 "IdleByte 2"){kind=icon}
 "Defender Idle"){kind=icon}
 "Idle Breakout"){kind=icon}
 "Wizard (And Minion) Idle - Beta version"){kind=icon}
 "Prestige Defence"){kind=icon}
 "Gragyriss, Captor of Princesses"){kind=icon}
 "Stop the Darkness"){kind=icon}
 "Idle RPG"){kind=icon}
 "Grow Defense"){kind=icon}
 "Aisleen"){kind=icon}
 "Idle Earth"){kind=icon}
 "Bubble Tanks Tower Heroes"){kind=icon}
 "NGU IDLE"){kind=icon}
I may just keep it free, or have it cost gems to use or neither! Listening to the community.