Hi. I've gotten most of the achievements, but am lacking Minimalist.
I've met the requirements several times, by playing advanced (not first 3) levels with only level 1 towers. I did not get gold, usually, but the requirements just say you have to "beat" the level.
Lately I thought maybe you have to actually play in "Green towers only" mode, so I tried that and beat the level, but still no achievement.
I'm not earning multiple achievements at once, so it's not that Steam bug.
I have had some problems in the past, e.g. earning the Shell-shocked achievement, where I had to earn it several times before it would pop up.
I'm using Steam... connected to the internet most of the time.
When I press Alt+Tab, it shows me the list of achievements I've earned, and it has worked recently on adding several new achievements. So it's mostly working! It's just not working for Minimalist.