I like the different possibilities to win a map. One of the best TD on Kongregate! Hint for winning the later challenges: The fire mage tower is very powerful!
Cave 1 - 3 be prepared for 1 demon after general kill - kill demon and rush the castle
Cave 1 - 3 (easy with full developed units, hero and spells)
Cave 4: play very defensive (full army count/stay in your castle/lure the general with comet) - be prepared for 3 demons at a time - after slaying all 3 - rush the castle
Cave 4: (some skill and luck needed with full developed units, hero and spells)
Cave 5: just wait until your units count is full - attack with full army - try to rush the castle with all you have - use spells, when you are next to castle (easy with full developed units, hero and spells)