There is a discussion about mines in the ocean but what about land mines? I know they were used extensively in the African campaign and I'm assuming in every other theatre. They exclusively helped the defender so my suggestion would be to make them a province improvement like any building that give maluses to attackers. They would not be large maluses, they can not be repaired(they can be rebuilt using IC), and they are fairly quick to build and a bit expensive to put up to prevent spamming.
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