Having now cleared Blood on King difficulty, I have a few suggestions. Maybe they aren't good, I don't know. I'm not a game dev.
- blood could use another permanent non-gimmick unit. maybe some kind of fragile ranged attacker. alternatively, make altar demons a standard unit and not a building.
- the imps spawned by blood's castle need to draw aggro. as it stands, it's very common to lose a life turn one because the enemy army's units can walk past your castle imps, and the imps are too slow to catch up.
- the library+spark combo should be toned down, or other buff cards brought up to its level. blood's buff cards in particular feel weak.
- on turns where you can choose a perk, consider having the perk be chosen after a card is drawn - feels bad to take the +1 to all tiles perk only to draw a tile immediately after
- allow me to spend gold on more things. i can reroll shop inventory, why not end-of-turn card choices? perks? what kings i'll be fighting after the first cycle?
- the spells card that levels up a random tile when discarded is a very cool effect and i'd like to see more like that. maybe blood's demon altars or cannons could scale off discards as well as dead units?
- PLEASE consider adding a quick-restart button. it takes a deceptively long time to quit one game and start a new one.