This game is great example how to achieve great effect with minimum means.
Mild spoiler below:
It's not explained anywhere you should escape or rebel against your masters and it sort of tests you the player on how docile or independent are you, how soon do you start to question what is going on your small never changing salary despite enemy amount ever increasing and your masters always partying. I love this moment.
It took me a week of two hours daily to get the best ending which is really hard but really satisfying.
To explain endings once fully
#1 Lose as an obedient wage slave. (easy)
#2 Lose as a rebel. (easy)
#3 Buy the key and escape the gate. (medium)
#5 Destroy your masters, but remain trapped in the area. (hard)
#4 Ultimate ending. Buy the key then destroy your masters, then leave with freedom and glory. (very hard).
On first two dollars buy shield.
On next two dollars buy speed.
On next seven dollars buy key.
Buy up to three health before rebelling. Never buy your last health you should get it from a heart that drops from sky at later rounds. But the truth is you will not likely complete the game if you lose any more then one health before you rebel, because each buy costs a round and later rounds just get harder.
Decoy might be useful but I find it too confusing and random, not needed.
Bombs are useless.
To gain easy advantage at late game rounds destroy the last enemy of each wave near castle wall. Turrets work best for this because they cannot move. Then enemies will spawn all around you and you can take two or four of them out in your first moves before they begin firing. Just get out of being surrounded fast.