The basic rules are:
You play as the red mage and your goal is to defeat the blue mage.
You and the computer play one card each turn (and draw a new one at the beginning of the next turn).
For each card you have to pay bricks or weapons or crystals in order to play it.
Cards for building you castle usually cost bricks to play.
Attack cards cost weapons to play and crystal cards have varying effects.
If you have not enough resources, you can discard a card.
The number of builders determines how much bricks you get each round, the same applies to soldiers - weapons and magi - crystals.
You win if your castle reaches a height of 100 or the enemy's castle is destroyed (height = 0).
You lose if your castle is destroyed (height = 0) or the enemy's castle reaches a height of 100.
Thanks for playing!
Update Version 1.1

replaced total random card draw by adding probabilities for each card and also a card is never drawn as long as you already have it in your hand
you can now discard playable cards by pressing the down button when the card is selected
made some of the cheap attack cards stronger to do more damage
mages on the title screen look a bit more badass

If you wish you can still try out the old Version 1.0: