Numberofilia (extra large numbers without meaning), the variaty of enemy's are higher then the units you got. You got zombe, zombe in skeleton skin, zombe in ghost skin,.... the first non zombe is the flying skull. After some level all the enemy do aoe damage you dont. Even the cannons do aoe. The numberofilia kills the challange 10 level lower you kill without loss, 5 level higher you got killed without chance.