Heroes and Castles (iPhone iPod)
CheatingDome.com
Various Game Hints
Level-up your initial, cheaper units first, as you'll need to rely on these often throughout the game - especially early on.
Many enemies will be distracted by your presence, so try leading them on a merry chase away from your castle.
Don't be afraid to retreat behind your walls to heal. If you play as the dwarf, you can even shoot through the door.