Enter and retrieve the gold from the dungeons of the wizards (blue, black, white, red, green). Exit the building, and re enter it. Upon reentry, the castle will have changed, but you will have more gold in it.
Defeat mages quickly in duels
When you play Shanadalar, if your deck has the card in known as the Black Vice, when you put your first Mana out, play the card. That will help damage the enemy enough to win without even bothering to play a single monster. The other way to defeat them is to use the Cursed Rack, but then you would have to make sure that the enemy puts almost all their cards down before they can receive any damage.