From within the game, press CTRL+SHIFT+~. This brings up a small text icon in the bottom left of the screen. Then, type ''@ '' (without the quotes, but WITH the space) and any of the following cheats.

alliwantforxmasisa - Summons one monster of the given type without using a soul. Ex - @ alliwantforxmasisa firefist
aplethoraof - Summons four monsters of the given type without using up souls. Ex - @ aplethoraof firefist
bythepowerofgrayskull - Full heal
ihavethepower - Full mana
dontfearthereaper - Gives you 32 (I think) souls
castratetheheathens - Wizard can collect red spirits
gimmegimmegimme - Gives the wizard a spell of the given type. Ex - @ gimmegimmegimme erupt
timeisonmyside - Resets the spell timers
yourbulletscannotharmme - The wizard (and only the wizard) is invincible
mywingsarelikeashieldofsteel - Same as above
ragebuilding - Levels you to level 9 and gives you all the spells and creatures accordingly (only in local multiplayer)

Hints:

Assign key bindings to all commonly used spells trying to click on the spell tab and find the right one to cast takes too long in the heat of battle.

When you have enough souls, be sure to issue the Guardian spell to two or three of them to protect your altar from attack. They might not be enough to beat back an attack, but they are likely to hold off the enemy long enough for you to teleport back with reinforcements.

Learn the special abilities of units and take advantage of them during battle. A Druid will use his Life Shield on his own accord after he takes damage, for instance, but he would have come out much better if he'd waded into the fight with the shield already active.

Take advantage of computer allies when your force is small by positioning your creatures near them during battle. They'll divert some of the attention away from you, and you can collect the souls of enemy creatures they kill.

Once you have two or three Manaliths erected, start building shrines at mana fountains as you advance farther from your altar. Sac Doctors can take souls there, reducing the risk of being killed as they make the long trek back to your altar