I was playing a game tonight with a friend and it came up that he had plenty of resources to burn when he attacked me, so he spent two resources over and over again to give his guy and additional hammer each time. I didn't see anything that contradicted this in the rulebook, so we decided he could do it. I just wanted to check and see if we played this right.

We had the same thing happen the other night, and we voted the same way, that there was nothing to keep a player from stacking power on an attacker using the Runesmith.

Berc

If it works that way, I believe a runesmith or two is in dire need for my Empire deck.

Everything in W:I stacks unless it explicitly says it doesn't or it is an effect which just cannot stack because of wording. (For example an effect which ignores the first damage inflicted on something couldn't stack since there is only ever one first.)