This is probably simple question, but as I just started playing I figured I'd ask so I can know for certain the answer when I start teaching the game to others. If you are playing a 0-cost Steadfast card, the Sword of Y'ha-tallo comes to mind, and the only domain you have with the necessary 2 Cthulhu resources is currently drained can you still play it? Seems like you should be able to if I'm reading the rules correctly...

You are correct. Enjoy.

One thing to remember regarding Steadfast - the required resources don't have to be on the same domain. For example, with the Sword of Y'ha-tallo you could play it if you had 1 Cthulhu resource on domain A, and 1 Cthulhu resource on domain B.