The current version (1.16.4) has a bug when playing castles - the hero’s attack/health stats and skill tree (decoy, shield etc) and possibly also the level bonuses are taken from the character selected in the main screen rather than the character you are playing with. The passive skill (snowball, stress shot etc) is applied correctly.
This is likely to be patched soon, but in the meantime if you are struggling to progress in castles, selecting your strongest hero in the main screen will give you a boost while playing your other heroes in the castle.
If you don’t want to take advantage of the bug, make sure your main screen is set to the character you are playing with.