Placing a wall near each GM is a great strat for norse, especially if you're going with a two Ulfsark opening.
Walling it all off is probably more time-consuming than it's worth, but if you have full map scout and you notice that they only have one or two expand GMs, it would probably be worth it to wall it off. If you wall it carefully, he'll have to destroy at least two sections in order to mine efficiently.
But you can get the main advantage for just dropping one tower: You scout the GM. Your opponent cannot use that GM without first destroying your wall section (or letting you know that 100% for a fact, he's mining there). Which means that they have to send troops out to destroy it, if they want to keep you in the dark. If you're Norse and into the whole raiding thing, they might need their troops back home.
Annoying, but a legit strat.
More annoying would be walling you in or something. Not really practical, but if they pull it off, it could screw with you (not to mention screwing with your mind).
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