Each player starts out with one castle on the game board. Your objective is to capture all your opponents' castles, so that you are the last player remaining in the game.
This game in the Risk family of games is markedly different than the original Risk. The combat mechanism is the same but players receive armies at the end of the turn, not the beginning (the sole exception being playing a Reinforcements card). The game has no territory cards, and one does not get armies based on the total number of individual territories. One gets armies for regions controlled and banners in one's castle only. Each player starts with a single banner in his castle but also takes the banners of any captured castles.
The cards act as die modifiers (General and Marshall), allow attacks one would not normally be able to make (Admiral), get armies at the start of a turn (Reinforcements), force temporarily non-aggression pacts (Diplomats) and look at another player's cards, discarding one in the process (Spy). The Spy card can also be used to defend against another Spy.
Rules Summary of the game in a 2-page format. It does not discuss how combat is resolved in the game, but it does address areas where cards are involved or when attacking Castles. It is assumed the reader knows how the attack rules of Risk work. It also has sections on how Castle Risk plays differently then regular Risk.
Note: it features two areas where the rules are not clear, and if I find out the correct ruling I will update this document.