This is probably the most fun and replayable game this jam, just due to the shear amount of playstyles and choices available to the player. There are a few little things that make the game less fun than it could be, but for the most part the gameplay is entirely designed around you being able to create your own unique formations and strategies.
I think the main thing that contributes to this is the fact that cards abilities apply universally, with no exceptions. If the frog says it heals the castle when it dies, it heals the castle when it dies, even if you yourself killed it. If the cultist says it deals 1 damage to all enemies when you draw a card, then it does so; even if you cast a spell that makes you draw 3 cards, it deals 3 damage to all enemies. And this strict adherence to the rules can lead to some really cool and creative unit combos, like placing a scholar just to get its card draw, which will make a cultist attack, or placing a bird to intentionally be killed and get its card draw, which will make the cultist attack . . . yeah the cultist is kinda just one of the best units. The synergy between units can lead to some really deep strategies, like holding on to a bird even though you will take castle damage, since it will have more damage potential by dying on the next turn when there are more enemies for the cultist to hit. Or like placing a treent to specifically spread its higher damage to all your weaker units using a spell, then being able to catapult one of the lesser units that now has decent damage.
Another thing that contributes to the fun is the fact that for every attack the enemy can make, there is (almost) always something you can do to counter it. If a high-health skeleton spawns, you can hit it with multiple damage spells or draw cards to make cultists whittle its health down. If a high-damage skeleton pops up, you can exhaust it using a spell or a bear or shield-bearer to prevent its attack next turn and buy yourself some time. If you feel like a ram might appear, you can place a new unit, as when it's exhausted it will be able to take whatever damage the ram might do and then you can exhaust and kill the ram. Admittedly the rams are probably the least counter-able, though I suppose without them you could probably find a way to just carry on for a really long time without losing.
The last reason I'll mention here for the game's replayability is that on an idealogical level, the enemy skeletons can represent death and decay, which is something that humans feel sort of innately driven to fight. We know we'll lose against it eventually, but something about putting up the best fight we can while we still have the strength is just so invigorating. I'm probably reading waaay too much into this, but my point is that if you give someone a game where they fight the embodiment of death, and you make the game fun in other ways on top of that . . . it ends up being pretty fulfilling (though I guess the enemies being skeletons doesn't really change the actual gameplay much; they could be unicorns for all we care and the game would play out pretty much the same).
Now . . . I've heard something about there being a boss? I have yet to make it past level 90, and I'm assuming the boss shows up around 100 or so?? Anyways I can't comment on the boss fight if there is one for that reason.
So yeah, there's a whole bunch more I could say but I think this is probably enough for now 😅
I had a lot of fun playing this, so congrats to both of you guys, and hope to see the game updated