After this season I’ll have enough to upgrade one, I’m leaning to defense. Because if the opponent has a bonus over you, you usually get your defense destroyed easily. Somehow I fought a defense with a level 1 fortress in tier 17. It would have been challenging otherwise.
I opted to upgrade my Defense Fortress first. With smart play, winning on offense is easier then defending with AI. So you need all the help you can get in that department.
I've managed to get defense wins a lot these past few days, already dodged dropping lift 3 (!) times. Thats 240 lift! I'd have to lose one party member on offense 12 times to match that.
Actually, imo is it is really up to you, if you are facing difficulties in your offensive team as you are unable to handle certain units like highly merge surtr or this season bonus units aren in your favour then i suggest upgrading offensive fortress. If you are fine with your offensive team, then go for defensive fortress. I too can upgrade one of them but i will look and see the bonus units for the next 2 seasons and decide whether should i go for offensive or defensive fortress
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