For some reason instead of carefully thinking where and when use my cards I spammed them because I had just too many of them. Also some sort of a pause to read the text the cards would be nice.  Glad you  added progression by adding rooms over time - nice touch. 

Here is my first attempt:

The game looks so beautiful and polished, I  really wanna play it more, but I couldn't put up with the card mechanics.

• It's a fast-paced real-time card game, but card text is lengthy and hard to read (it's a tiny pixel font), so inevitably I lost a lot of health just trying to read card text and grasp what it did. Make the text much simpler or make the game turn based, pick one.
• Give the player a timer at the beginning of the game like in tower-defense games, so they can prepare for the onslaught (and read those lengthy cards).
• If I want to pick a specific card, they are too close together, so often times I grab the wrong card by accident. Give them some space, so I can reliably grab a card if things get heated. It's not like you'll get 10 cards in your hand at any point anyway.
• Because they are so close together, it also makes it much harder for me to see which cards I'm currently holding, meaning I gotta scroll my mouse across my entire hand pretty regularly just to get the basic info needed to form at least something that resembles strategy.
• On the topic of cards, at one point I could only grab them from the left side or by the icon instead of center mass. That could be a bug, but it was very detrimental to the play experience and one of the initial reasons I stopped playing before giving the game a second shot.
• If you're going to animate the cards, don't also animate the collisions. If I hover over a card, the animation can make it possible for me to choose a different card at the last moment, because the collisions moved. Same if I want to grab a different card instead of the one that is currently huge.
• Thanks to those same animations, a bug can appear where the card stops becoming selectable unless I re-select the card. Again, frustrating. Those could also contribute to why the game got picky sometimes with grabbing cards.
• When placing monsters, the placement is canceled automatically if if touches the wall at least a tiny bit. I think the game should help you here by moving them out of the collision and into a proper place, which should be easy if the level is grid based. Only cancel placement if I intentionally place them somewhere dumb, here nothing can fit.
• Small touch: make the corridors red when grabbing a card, letting you know immediately you can't place them. At first I didn't know why I couldn't place them until I realized there is a monster limit per room and corridors are purely connectors.
• Late game, the enemy swarms got so huge, I just started spamming all monsters and spells, since a single monster couldn't do anything against 4 knights and spawners work too slowly to act as a deterrent. I didn't feel like the fight were fair. I think slowing them down could not only make late game a bit more lenient, but also make the early game more inviting.
• Maybe monsters from spawners shouldn't die if the cooldown runs out? I want to use them when they're at their most effective (away from the enemy, ready to build an army), but instead I'm forced to use them when they aren't useful at all, next to the enemy where they die instnatly. You have cannon fodder enemies, but you won't let me play them as such - they're treated instead as singular, stronger enemies (which they aren't).

I sound very harsh, but that's because I wanna love this game so bad and play it, but all these issues prevented me from even penetrating the game and giving it a fair shake. The game was constantly giving me reasons to quit. It's a card game where using cards is not fun or easy, and if that isn't fun, there's no reason for me to keep playing.

honestly, this was a very fun playthrough! I am truly impressed!

firstly, the game looks so nice!
visually it is excellent I really dig the colors and the pixel art! the tiles and dungeons are so cool! the game is very matching thematically thus leading to a visually pleasing experience,

game plays well overall, controls are solid, and the gameplay loop quite satisfying too, it did not get stale as fast as I thought it would! so my experience was quite fun!
on that note I found the game quite hard too, I was overwhelmed easily and it took me a few tries to understand what is going on and actually enjoy the session.
the concept is quite smart and well executed so well done on that too! great work especially for a such a short duration!
the game has quite the polished feel thank o its excellent visuals and solid gameplay, my only small complaint is the balance, the game needs a bit of nerfing on the difficulty and perhaps a small guide to help players, other than that, the concept has a lot of potential, I would totally play something of this nature in a more expanded take,
overall a very solid entry, and a memorable one thanks to the visuals, the concept and the charm of the game, great work!