(continuation from my previous comment on strategies) Make sure you have soldiers/archers/mages ASAP, first to the Warrior and the Horseman, they can't defend against ranged enemies until they are close enough. The Sorceress can defend herself, especially if she's well equipped, so give her at least one archer and one mage just in case she gets overwhelmed, she has some back up. Don't replace/upgrade any of the guards, its better to have more in numbers than less with more power. Once all the slots are filled, then you can start upgrading the guards. ENJOY X]
For those who say that their Warrior and Horseman dies, here's what I use and they do quite well, even during a massive attack
Warrior: Maces and Light Armor
Horseman: Spears and Cloaks
Sorceress: Rods and Shields
(most weapons and defenses will be unlocked throughout gameplay)
When buying items at the store, look out for the ones that appear with "random magic bonus". One is alright, recommended buy the one with two. If none appear, exit and click on "HEROES" and the store should have a different inventory. try and keep ones that the magic bonus contains either "resores X HP in X seconds" or "returns X% of damage to attackers". when newer weapons and defenses appear, buy them ASAP! The newer equips have higher HP/Leadership/Damage/Damage per second.
Not to say that this is a perfect strategy, but if you fail one mission, u keep the gold, treasures, and especially the EXP, go back and fight again and you shall be successful!