Not an idle game. You have to be paying attention for your tower to advance. The game is an interesting strategy so I keep coming back to it, but I'm not aware of any means of having it run "offline", and it will even explicitly stop certain processes if you lose internet connection. So, absolutely not an idle game, but okay-ish outside of that. I legitimately wish that my neuroses about "beating the puzzle" would let me walk away, as I often find myself feeling obligation, not fun.