Steam is down for routine maintenance and this happened

I clicked on cancel and it closed the game
So why does the game need constant internet connection even in singleplayer?
Yes, I was in singleplayer.

you have to select to play in offline mode in Steam. this is a Steam process for game/purchase verification, not a ST thing. any game will do this if Steam is trying to connect.

I agree that this is really annoying. There is absolutely no need to constantly check for a Steam connection. Especially not in single player. And no, this is NOT a Steam thing. None of the games in my library I recently played does this.

This IS a Steam thing... Run Steam in "offline mode" and that message won't appear... Some games just handle being disconnected from the Steam service silently instead of showing a message like this. Guess you could say it's a "group effort" in this case...