Make sure the primary and secondary colors of energy that your castle produces match at least some of the spirits in your dungeon.
If you play more than 2 colors, the energy has to come from somewhere. Make certain your dungeon includes some energy pieces of the colors that the castle does NOT produce.
Slingers, spearmen, archers, catapults and the ballista can attack from more than one hex away. Use some.
Minimize the fog of war by including scout-type units with a high "vision" rating. They can help you spot the enemy castle and avoid powerful enemy units. Get them in play early.
Try not to clog the entry hexes beside your castle with energy spirits and Defenders. They can be cast further away.
Whenever possible, keep your units in their home terrain. Energy pieces will produce +1 energy, combat spirits will do +1 damage, and all units will have +1 protection.
Pay attention to movement cost, maintenance cost and resurrection cost. Budget your energy wisely. You may not want to cast everything in your courtyard at once.
Keep your energy pieces out of harm’s way. Consider casting a Defender near your energy pieces to protect them from fast-moving enemy units.
Most spirits cannot attack on their first turn. Those with the "first turn attack" ability (example: Spartan Militia or Praetorian Guard) are able to rush out of the castle to defend it. (On a similar note, be aware of those spirits with the "move first turn" ability.)
Killing the other player’s pieces can be fun, but remember that you have to destroy his/her castle to win the game.
If the situation is truly hopeless… the "concede" button is located in the top left corner of the screen!
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