“defence” works like this: when taking damage, defence is subtracted from the damage, and spent at a rate of 1 per damage prevented, but only if it was >0. So if defence = 3, and incoming damage = 5, then incoming damage becomes 2 and defence becomes 0. If defence = -3 and incoming damage = 5, incoming damage becomes 8, but defence stays at -3. I will try to re-phrase the explanation.
one guy who specialized in game design told me a good rules are elegant, which means the rules should be really simple to understand but hard to master (for example chess most 6 year old will understand the rules... but they probably won't play good chess) - in your game i feel like there are many "complex rules" that require some learning curve before at least me would be fully accustomed to them. The self melee and this defense rule i would put them inside this category - you implemented them for a reason - you probably like the idea of having negative defense (meaning the player suddenly taking damage) or the self harm "which can be prevented" when it is "own melee damage" but can't be prevented, when it comes to card cost - for me both of them break my basic "intuition" about how things are supposed to work in any game i played so far "defense was a good thing" and "harm was harm no matter which source" - for example after I noticed "I can't prevent harm from the cost of cards" I didn't give a second thought about preventing any harm from any card. What was giving me a hard time with understanding the defense rule is... that I tried to understand it without the "defense" you printent infront of the definintion... if you would have written "if the defense value is greater than zero an icoming attack will be reduced by..." it would have been easier for me to follow... for the sake of clarity i would probably choose a different word than defense though maybe something like "physical resistance" and then formulate it with a text rather than a formula and highlight some important aspects with color - " a physical resistance value > 0 will offer you protection, incoming damage will first reduce it to zero before you take real damage, be aware though a (red) "negative resistance value" will be added (red) "to your damage taken every single round"... maybe something in this direction if you want to go along this path. I personally think there was a reason why the games I know though, would not "mix defence" with other status effects, first basically to build up on "player intuition" and second to keep the complexity low instead of having a more complex "if then" rule you can achieve something similar by just having to simple to understand status effect like "injured" and "defense"... don't know if there would be fix that would you allow to incoperate this two status into your game by keeping everything more or less the same just to clear things a bit. ... I must say despite all the complexity i had some fun playing the game (just it was under all this clutter of many times "too much text to read"... which probably essential to do, to play your game really well" and for me being it a bit cryptic to understand... I'm used to some card games which I played quite a bit... although I'm for sure not the greatest of all card game fans... i played slay the spire for example and enjoyed it, but it wasn't my favourite game... just there i felt every card description was very self explainatory) In your game i frequently stumble over cards "which talk about status effects" I haven't even aquired yet and then it feels a bit confusing for me right from the start) ... and of course in hinsight having something to "push through" the deck fast to get more value out of bash makes a lot of sense.
The toughness does reduce card cost, so 6-cost Scales card pays for itself if you play more than 6 total cost of cards afterwards. Playing Scaled Skin + Muscles gives your 2 strength, which you did manage to pull off once. This combo is powerful but very expensive, and default healing in the deck is not quite enough to support it.
(yeah i felt not enough healing was a big issue)... didn't fully grasp the concept of toughness though... (it felt kind of expensive to pay for it... so i didn't want to invest too much time into fully understanding it)