Great!
3 plus:
-Music/graphic/setting merge together very well.
-Nice skill and level upgrade set.
-Special missions where we can revisit the same map but doing different tasks (for example the one where we don't have to kill goblins).
3 minus:
-IMO the game is missing a special feature that only this game has. There are many nice feature that are present in other strategy/defense games, but I cannot say this game has an innovative feature that I will remeber the game for.
-I'm not a big fun of strategy/defense mixed with clickers, so for example I didn't like the amazons because I had to go around the map and quickly click (often missing in time). The wolf can be fine since it's only one (it's like an extra skill, maybe it could have a button associated to it). The amazons were fine in the mission where we have to leave alive 4 goblins, but only because the main focus was on those.
-The level tree was not very clear. Maybe a tutorial in the final version will make it more clear.
About how to spend gems...a few examples:
-Skill reset (removing the free reset as it is now).
-Permanent or temporary exp boost for troops.
-Cosmetics (but I'm not a big fan of it)