There are two resources, gold and mana. Gold is used to build units, buy heroes, cities and spells (in Wizard Towers), upgrade cities. Mana is used to cast and research spells and use altars. Units can be built in cities, which can be further upgraded to produce more advanced units. Also, wandering "independent" (not belonging to any player) groups of units can be hired, if they are well-disposed towards the player. Units may have different strength, reflected by their "level", which can vary from 1 to 4. Units under player control require a certain amount of gold every turn as an "upkeep", the size of which depends upon the level of the unit. Summoned units use mana for upkeep instead of gold. Units earn experience for killing other units. The amount of experience earned depends on the number of enemy units killed as well as their level - the level of the enemy killed is the amount of experience gained. Upon earning a certain amount of experience, the unit gets a "silver" medal, and later on a "gold" one. Higher level units require more experience to earn medals. Units with medals also get slight increases to their basic parameters, or otherwise improve their combat performance.