population 600+ 
You cannot lose once you reach 200 population. I got bored and decided to use all the remaining cards (over 150 after completing the castle), so I started making mini islands for fun and stopped collecting new cards. i.e. put houses apart, etc. I was only collecting money. Finally cards ran out at population 600 and still got money lol.
pro tip placing grain around the mill in circles is OP and will give you a lot of money.
nice game, I wonder what they will add in the full version
(the screenshot is 4 merged into one, dont know how to zoom out)