You can recruit heroes at the tavern. A better tavern will attract more and better heroes. Upgrading the Tavern significantly affects the quality of recruitable Heroes. A level 10 Tavern will occasionally have Green or Purple Heroes for hire. Obtaining Green or Purple quality Heroes is possible at lower level Taverns, but far less likely. Upgrading Tavern level to 10 should be a top priority.
Each race has a slightly different tavern.