mana as it is right now is an absolute pain in the ass (though I understand why it's needed) and make some parts of the game horribly slow/hard due to the AI doing whatever the hell it wants, would love for a way to refill some mana during long fights, so they don't have to be so frustrating?
maybe make it refill only to a certain point (read; only one turret for example)? you could make it so that it will only refill if you haven't been hit for X seconds to keep it somewhat challenging and not make it easy to abuse against hard-er fights? or that and also having to have no turrets in place right now, you'd then have to let go of your old turrets (that are probably obsolete now) and then wait for the mana to come up to finally make a new one, one that you could finally use to kill the thing that's been hiding in that corner without having to waste all your health doing so
that would also mean a way for players to not waste mana on destroying poop in monsterless rooms for that one last penny they could need, which is one of the most frustrating parts about having a character that doesn't have his 'own' tears

of course playing without the mana (or modifying the files to get more than enough to go around) is a way to go around it, but then it becomes too easy, for obvious reasons

have you tried implementing a dynamic mana regeneration system instead?
turrets would only last X amount of time (could also scale with a stat? or not) and your mana would keep going up, of course you'd have to reduce the maximum mana (or make turrets cost more)
I think this would translate very well here, no more frustratingly having to wait for that enemy to walk in front of your turret fire

also I was kind of disappointed about the turret shots not synergizing with multiple shot upgrades (like quad shot) or some charge items (like monstro's tooth), though at least it seems like the damages still scale so, I'm not going to whine too much about it!

absolutely great work otherwise, very great idea and good fun (when it 'works', at least ;))