AgusBocassi, they reduce one of your stats (Str, Agi, Int or Wis). Some of them affect on several characteristics (Weapon, Magic, Absorb). So if you've got -1 Str, you can receive -1 Weapon and -1 Absorb.
Best build to beat any level. Huge group of units in the center of the map: David (slow upgrade), Billy Kid (stun upgrade), Tecumseh (visibility upgrade), Thomas (splash radius upgrade), Chuck (scary upgrade to make him attack target with full HP). At the beginning of the path: Billy Kid with headshot upgrade. You can use Johny as a "cleaner", placing him last with splash upgrade.
Can't agree with that units should receive additional damage when leveling up. The main thing in this game is managing units' bonuses, not making huge amount of damage.
Frankly speaking, damage upgrade is useless, because it's cheaper to buy new unit. That's why Johhny has no future: lack of good skills. Chance to miss is not a great invention too. It's sensible to keep this chance for Chuck (lol), because of his ability to increase attack speed with each consecutive shot, other units should have 100% accuracy.
I think, that there should be some kind of "spell": trap or something with a low damage to stop one bandit, who have bursted through your defense with 1 HP. And the last thing I want to mention: Billy Kid is too strong, he can attack 2 targets at a time, stun and has a chance of instant killing.
Great conception, enjoyed a lot. Hope, there will be sequel with more skills, levels, possibility to remove the scenery and better balance.
5/5, thank you for one of the best TD on Kong.