Still, it's one thing to get 20 defensive points, and another to get 40, plus to gain +2 quarry, which means you can build up your wall even more. Stormwind is not overpowered, because by increasing Magic, the player won't be advantaged in a defensive aim by gaining additional bricks per turn as in this card here. Also one more thing, +40 wall is extremely strong in terms of defending against Charge cards. Stormwind doesn't give you that perk with +20 tower, while this one will basically cripple the opponent in most ways, unless the opponent is able to counterattack with strong -wall cards. But in the meantime, you can build more your wall using the +2 quarry. Quarries cost as much as every other facility, except that +quarry cards mostly use bricks, which are not so important as they can accumulate or increased fast, and some gems, while other +facility cards use mostly much precious gems, which are also needed to play a plethora of other cards. On the other hand, +dungeon cards also use some gems or bricks, but use mainly recruits, which are also needed in a lot of expensive damage cards, if your deck is based on recruits, and recruits are not so easy to accumulate and/or increase as bricks. Even in tower building decks, bricks are almost never a problem, since almost every card you use gives back bricks, Unliving returns a large amount of bricks and there are also a lot of Brick increasing cards that can be used additionally.
In other words, this card is way more overpowered than Stormwind, as it gives you a huge defensive gain, useful against anything and especially against decks that rely on Charge cards, which can also last for the long-run due to + 2 quarry, while Stormwind won't help you any further when mounting a defence, but it provides you with extra Magic in order to mount an offence (which means in the meantime, your opponent has a good chance of beating you => balanced card)
That's why I suggested +25 wall instead of +40. It's not that great increase, but it's still sufficient for a rare card of this cost and the +2 quarry will help you increase it even more, while in the meantime, you give your opponent a chance to retaliate.