The game is pretty hard (4 wins out of at least 30-40 attempts), but it's impressed me enough to get myself to give an advanced critique, though I'm admittedly not very experienced in this genre.
First is the difficulty spike that occurs when Snitches are introduced, at the same time when the amount of enemies in general also spikes up significantly. (turn 4/5) I feel like that should be delayed to when you get Tier 3 units. There may be one on the turn that Knights and APCs get introduced since dealing with 1 is already tough, but a double Knight + double APC one is pretty much invincible. It'd be good to note the concept of Unstable Equilibrium since getting a bad round early on is going to make your chances of winning more bleak. The other part is feeling like coins are too scarce in later game, assuming you're not lucky with getting the building that gives you coin generation. Maybe nerf the coin gain for upgrading that building in exchange of extra coin gain at later turns? Or give more units that could increase coin gain.
Second is that spells don't seem worth playing around with unless you have a solid defense already. It's mainly because most losses are because you've basically let the enemy scale up too long, and spells don't consistently help against that compared to another unit that could do more damage to the castle. (Maybe make the king a little stronger at early stages but lower the castle HP instead?) Bat Curse feels meh at early-game, almost useless mid-game (where I'd say things get shaky) and niche late-game for removing shields from Knights. It doesn't synergize well with Bowies and Wizards. Skeleton summon feels ok, and the Blessing one I feel is mainly good for clutch victories. Personally, given the disadvantages of spells, I would make spells cost 2 coins instead of 3.
Third is whether a way to "banish" a unit for a price would be viable to add.
As for specific balance tweaks I'd recommend as well:
Maybe to ease the difficulty spike that a group of Snitches can bring, maybe lower the amount of Snitches but add a new Priest enemy for early game with 2 atk + 1 hp that can also heal enemies in front, which would pose a similar threat without taking 3 hearts if it gets by. (This may hurt the viability of Vikings, whom I feel need a buff the most out of all Tier 1 units anyway)
Wizards feel very underwhelming for a Tier 3 unit, given that it is essentially useless without spells and unless you're lucky enough to stack spells early, is an inferior Bowie. Also, the Wizard doesn't seem to be able to store multiple spells. I would either give it a damage buff + inflict Bat Curse at Lvl 3, or demote it to a Lvl 2 unit. I think it would also help to add another unit that benefits from spells by buffing itself on spell to encourage a spell-friendly build, like some magic golem.
Summon builds feel powerful if you get good rolls for them early on, even though they're difficult to set up, because they also have the benefit of hurting castles more. Maybe give some other units +1 damage to castle?
I'm conflicted on the Big Boned, because although it can get broken with enough support, other units like a buffed Sticky or a Pigman feel more reliable. But as a Tier 4 unit it doesn't have the tide-turning feel every other T4 unit does.
Excalibur doesn't really seem to be worth the buildup because its health is low enough that even with a shield and milk, a Snitch or APC will halve its lifespan. Personally I'd let the Lvl 2 to 3 transition double the attack and health, but maybe you have something better in mind. (Or maybe more front-line summons are needed)
On a final note, if you feel like expanding the game, I think a simple but neat addition would be different playable necromancers with different bonuses each.
