Heroes that cost more gold are more powerful and rarer – there are only 6 heroes that cost 5 Gold, with a limited stock of 10 units for each. More expensive heroes get combined/upgraded the same way as less expensive ones.
Heroes have synergy on three levels: Race, Class, and Abilities.
The general logic for Race and Class is that more of the same is strong. For example:
- Race: Having (2) Orcs on the board will increase their HP by 200. Having (4) will increase their HP by an additional 300.
- Class: Having (3) Warriors on the board will increase their armor by 7. Having (6) – by an additional 8. Having (9) - by an additional 9.
The simplest strategies would be to stack one of the same thing (usually class). Going too far in one direction, however, has drawbacks.
For example, having a lot of warriors is an easy way to build a powerful army. However, Warriors stack armor. This makes them very vulnerable to Mages, who deal magic, not physical damage. At the same time, Mages are vulnerable to Assassins (because they reach them very fast on the battlefield and deal high damage), while Assassins are vulnerable to Warriors because of their high HP and armor, etc.
Because of this, to build a potent strategy you usually want to combine a few (but not too many to keep the synergy) different species and/or classes that complement each other.
E.g. your squishy Mages might want three Warriors on the frontline to buy them time. If two of said Warriors are Orcs, they would have not only more armor but also more HP.