The Big Wall seems very powerful and since I've started to use it, it's been very helpful. Generally I like to use anything that gets me more rolls or more dice. So Gwen, excalibur, cauldron, etc seem strong to me. I agree that spells feel a little weak to me right now, so Merlin and the spell book don't seem that useful. The market seems very useful as 1-6 gold isn't much to get the exact dice you need. Plus just building the market gets you a ton of stuff which is nice. I can't tell if Percival is good or not, he feels decent if you don't have walls since he can make all your army missions one kill stronger. Also he adds decent defense especially with armour.
Right now the most unbalanced things to me seem to be that Certain villains are way more annoying if not outright harder to face than others. Lancelot doesn't seem that bad to me as I don't use him a lot as a hero. Which ever one takes away one of your dice in the battlefield (I think Morgana?) seems to be the worst one as it straight up prevents you from completing 5 dice missions unless you have something giving you an extra dice (morning star, swords, etc). That seems way more damaging than any other villain like say the Black Knight who just kills one dice when he spawns.
The other big problem is just if you get hard to combat monsters as your first attack wave, that really screws you unfairly--like say Manticore + Gryphon.
My recent decent game. I don't know why, but I try so hard to unlock all the days even though you don't really need them.
Also, I'm not sure the brewery is giving me my gold for guys in the tavern. Do you have to use it on that turn to trigger that?