Beerse [near Turnhout]
In reading the posts it is quite apparent to me that some people are playing that you take cards into your hand and can even save them for future turns. I don't interpret the rules that way at all. At least in the English rules I find nothing that says the cards are ever taken into your hand. I am afraid you are wrong. I don't have the English rules, but the original German rules are pretty clear about this [page 4 - bottom of right column].
1. We are talking here about the "neutral" advantage cards [as opposed to the blue/red colored "personal" advantage cards].
2. Neutral cards can be used by both players. Amongst these are "1x" cards: players can put these before them and can play them any time in this round or one of the next rounds.
3. If you don't play the card before the deck [A or B] it was originally coming from needs to be reshuffled, you have to hand in the card "unused" [making hence the A or B deck complete again].
Also the English rules that have been posted by chicagometh explain this on page 4 - bottom of right column.
Also I disagree with the interpretation that some have given to the "Storming the Castle" card. A literal reading of the rules renders it only of practical value to the first player. Since there are several "Transfer of Power" cards, it seems to me that this is one benefit of going first. Those who are reading it differently are inserting the word "immediately" on the card which does not appear.
As further support for this interpretation, suppose the "Storming the Castle" card is the contested card. You have to resolve the contest before you know who gets to use the card. Once the contest is resolved it is then the first player's turn. I don't think you can allow the second player to take an action first!
Given the facts that:
1. You can keep this card from a previous round
2. You can play a card any time during your turn!
When you have this card as
a] First player [= owning the Power marker]: you can play it before the conflict OR even after the conflict has been resolved
b] Second player: no use whatsoever as the first player has already done his/her actions. Remember: one can only play a card during your turn!