Colonels

Colonels (3 (or 8 in the European version) are the third-highest-ranking piece, and you only get 2 of them. They can safely attack 24-32 of your opponent's pieces. It is generally best to keep colonels at least somewhat close to the front, in case you need them.

Strategies
Since there are 2 colonels, you could use completely different strategies for each of them, but they are generally kept for defense until the opponent's marshal and general have been defeated, and this is usually a good idea, as even if revealed, an opponent will likely not attack a colonel with their marshal or general if there are other pieces adjacent to it, and you can always place your spy and or marshal next to them, just in case they do.