How Do I Install Stronghold 3 Gold Without A Key Code?

Solution 1:

There is no way to legally play a game that requires steam, without the CD key.

If you purchased the game, new, return to the store with your receipt and demand an exchange for a copy that has a key or a refund. No matter where you are your consumer rights should at least cover you for that¹.

If you purchased the game, second hand, you might have less rights. Regardless, all you can do is return to the person that sold it to you, and demand a key or a refund.

_{¹ I assume, from the context of the question, that you have purchased the game within the last week. I expect you would need to return to the retailer as close to the purchase date as possible to claim not having included essential components. While warranty covers this inclusion, warranty for games can be different to other products. For example, in Australia most products come with a minimum 12 month warranty. Games come with only a minimum of 30 days.}