Nice very short demo.
Don't know if it's a bug or just not implemented yet but triple shot and dash are just not fonctionnal, plus you can buy them multiple time for no reason.
There is a bug where the "castle" in the top of the arena don't have hitbox so you can move underneath it.
Suggestion :
-Bigger monsters should drop more gems.
-More building to build in the arena like towers that can help you shoot the ennemy and that you could upgrade too, upgrading castle to get more soldiers, moat or ditch to slow down ennemies ect.
-More things to buy when you die to get stronger next time, like upgrading your attack speed, movement speed, shoot 2 arrows then 3 (should be very expensive), maybe spells or more weapons.
-Other than upgrading your heroes, upgrading your arena could be good, like "start the game with 5gems, then 10 etc" or "start the game with 1 tower"
-We could also imagine a mine building that you could buy in-game with gem, that give you 1gem/10sec, then 2/10 sec at level two.
-Maybe try to make ennemis a little bit faster, and simple particle animation when you shoot an ennemis if you have the knowledge to make it. That's "simple" and make the game much more dynamic.
-You could add boss that appears every 3 or 5 waves, when you kill them you get a random special bonus or object, usefull to give more replayability to your game
As a game dev, I made severals rogue-like games like this one, thoses are simple and non-original features common to many rogue-like games, but hey, if they are so common it's because they are good :)