If you took a trading card game like Magic: The Gathering, added some board wargame mechanics...
If you took a trading card game like Magic: The Gathering, added some board wargame mechanics, and threw in an online player-matching server, you'd have something very much like SpiritWars.
To win, you must reach and destroy your opponent's castle. Getting out some fast creatures to scout is a good idea. The net effect is to sustain the metagame of deck construction, in which you're always trying to find the mix of spells that allows you to win, while adding an element of planning and strategy through the boardgame elements that's lacking in pure TCGs.