The author of the book teaches how to win in/at casinos.
...how to play roulette in/at casinos.
He won a lot of money in/at casinos.
He made a fortune by gambling in/at casinos.

When talking about casinos in general, no special place, no special kind of game, both 'offline' and online casinos - do you say "to play/gamble/win etc. [in or at] casinos"? Do I have to make a difference between online and offline casinos, because online casinos are not "surrounded on all sides" (explanation in my practical usage dictionary)?

I googled several phrases. Combining "to win" and "casinos", "at" ist much more common than "in", but is "in" incorrect? Combining "to play/gamble/gambling" and "casinos", "in" is more common. Combining all of these with "online casinos", "at" results in more pages. The texts I have to translate contain different verbs combined with "casinos" in general and sometimes offline or online casinos in special.

Thanks a lot!