All of the rules etc are on the game's main page, not sure if you didn't see them or if they weren't clear enough. Either way: cards can only attack cards directly opposite them (in the same column), and only if the selected card's attack value (red circle) is greater than the potential target's defence value (blue circle).
If either condition isn't met, your card won't be given the option to attack. The obvious exception is when your opponent has no cards on the board, in which case all of your cards can attack the opponent directly regardless of position.
The yellow circle is the card's value, which determines how much gold you receive on killing an enemy card.
I wanted to get all this written out in a window in the game, but alas, time. :( Thanks for playing!