@ coins
What do you plan with it?
Now you can choose to use coins _or_ viewing an ad to be able to continue the game a single time. So you cannot spend coins _and_ watching an ad (to receive 2 continues).
An option would be something like
1st continue 50g
2nd continue 75g
3rd ... 100g
4th ... 150g
and so on
@ mother of all dragons
The first text line is done with a "typewriter effect". The second line is instantly shown.
When you reach second boss, same effect is used - while you already know what happens next (so instantly show both lines?).
Another idea is to "warn" a level before: "WARNING: next level is a boss level".
Boss levels could have a slight red glow at the border of the game (like a warning screen).
Next to boss levels you could have "difficult" levels every 4th wave - which means you have some dragons which need more hits. This also means that level 20 is a "difficult boss level" (4*5 ...).
Tower defences also had "invisible mobs" which needed special towers to get seen. Might be a bit too difficult ("needs level 3 arrow tower on each lane" or so) but just keep the idea in the backyard of your brain.
@ coins
For now you collect the coins "towards the hearts" - maybe animate the collection movement towards the coin indicator.
Right now it's competitive so I don't want to let people pay to win. Since gold is attached to your account not your current game. Everyone gets 1 continue.
I was thinking gold is stored in your castle and that's why dragons are attacking. But it's displayed on the right. And you don't lose your gold when they destroy the castle so idk.
That's a good idea for invisible mobs. I think I played one like that in a Starcraft mod where you had to build turrets to see the wave of cloaked wraiths. I forgot all about that.
Ya maybe it's better to say the mother of dragons is coming next rather than right now.