Okay so I have now beaten the game twice out of 4 tries.
First two tries I didn't have really any idea what I was doing and gave up after I felt like there was no way I was going to come back.
Win 1) I won without using any upgrades, I didn't notice that their were upgrades on the last page of the up/down menu system. Because of this it took a long time and was verryyy difficult.
Win 2) I spent some time poking through the menus and finally noticed that can upgrade your income/stockpile size etc. This win was very easy as I had tonnes of cash and good units.
I really enjoyed this game as you can likely guess by the number of times I have played it!
My main suggestions:
a) Improve the UI in the game to make it easier to see what each unit type does/how much HP etc. Also somehow tell the player that upgrading thier income is very important.
b) Just put all this info in the description. You already have a 'Tutorial' section, just go into more detail. I looked at this when I was initially stuck and I think if it had just said "Castle Upgrades are on the final page of the menu, these are very important!" I would have had a lot easier of a time.
Anyways, I had a lot of fun, just took a bit of effort, thanks for the game!