For a first game, it's really good, although there are definitely some ways in which it could be improved. I'll just rattle them off quickly – but it's your game and your decision if the feedback has any merit and can make your game better.
– It's pretty repetitive. I don't think it's very interesting to have to level up every archer again from scratch and by using a shared experience pool.
Then you're forced to make a decision between many average-levelled archers, or a single overpowered one and – as you said – the balancing isn't the strongest aspect of the game.
– Players probably shouldn't be in charge of advancing the waves by themselves. Leave some breathing space between the waves, but advance the waves automatically. Don't put the player in the position of "nothing dangerous happens, unless you voluntarily choose to" because a known phenomenon is that many players tend to choose the safe, anti-fun ways to stroll through a game without risk – which isn't very enjoyable in the end.
– The Restart button right in front of the castle is pure evil. If you need to shoot the enemies close to your gates, you could accidentally misclick and restart the game. Why don't you hide the restart button until the castle gets destroyed?
– Why are the enemies always spawned in a single giant cluster? Wouldn't it be better if they came in small groups in some order (for example start with 1, then send a group of 3, etc.)
– Are there different types of enemies later on? Not just with larger health, but with different abilities (for example enemy archers, or giants).
Also, if you don't know the game already, I heartily recommend you to take a look and perhaps inspiration from Defend Your Castle: https://www.newgrounds.com/portal/view/102209
– Enemies only harm the castle on the initial impact, then they just stand awkwardly until you shoot them. They should probably attack like every three seconds or so.
– You shoot arrows even when you click on buttons (which shouldn't happen).
– The arrow does crazy things as it passes through the ground.
Graphics & UI:
– It's a bit simplistic, but that's okay.
– The UI is somewhat chaotic and unaligned. The plus buttons don't have the same distance between them, there are different font sizes (and some are too small)
– Consider using icons for damage, speed and accuracy instead of text. Just a thought.
– If you can, avoid using the defaults because they are too recognisable and placeholder-ish. Default Unity buttons, default dropdown, default 2D scene blue background…
– Why doesn't the ground go all the way to the bottom of the screen? The castle and the thin ledge now look like it's floating.
Still, a good game, especially if it's your first one. Good luck if you decide to continue working on this one, or with your next one. And welcome to Newgrounds!