MArcomage

Look at this. Then this. Then this. Then this. After that you will be able to say what's wrong with this concept by yourself.

If powerful:
Great lookout towers

Far sight
If Tower < Wall and Tower < 45
Tower: +36
else
Tower: +18

http://arcomage.netvor.sk/?location=Cards_details&card=21 balance

If there were not the condition, the effect would be balanced.But since the condition is the same of lookout towers, we could balance this on this card with another factor for its rarity:

For determiting the factor,lets compare Stronghold and Tower:

Stronghold: +30 tower for 20 bricks +Farsight
Tower: +5 Tower for 4 Bricks (To replace the far sight keyword, lets take +4 Tower instead of +5)
We come to a cost : tower factor of 1:1 for commons and 2:3 for rares.

Now lets balance this card with lookout tower and the factor:

unconditional Tower+ of the common version: +6
unconditional Tower+ of the rare version: +9
Conditional Tower+ of the common version: +12
Conditional Tower+ of the rare version: +18

With this values, the cost/Effect ratio should be balanced

Edit: For higher Tower values, take a higher cost, the ratio of cost/effect is 4:3 unconditional and 2:3 conditional (Without the rarity-factor)

16/32 tower for 14 bricks with Far sight is already worth a rare slot, because rare Far sight is already 18 tower, or 27 wall, or 9 stock, or revealed enemy hand.