After playing for a while, I think EXP is ultimately king.

This is from the perspective of a free-to-play player who hasn't spent any money. I might spend money if I enjoy it more after playing a bit longer. As you progress, you need to level up your mercenaries for synergy effects. In my personal opinion, the top priority is a setup for farming EXP. Once you break through 1000 waves, an automation feature becomes available. You can upgrade desired features with gems, so it's good to save your gems early on. I don't know about other things, but for idle play, investing in auto-rebirth and hero leveling methods is essential. Focus on DPS along with infinite rebirth. It's recommended to achieve 1000 waves on the basic map. It doesn't matter which map, as long as you break through. Just unlocking the Crossbow and Druid level 15 synergy makes a huge difference in DPS. Global effects seem to be stackable. I haven't calculated it, but the DPS increases dramatically. You can only place up to 3 of the same unit. Level 15 is easy to achieve. The next synergy is at level 50, so it will take a very long time for a brand new beginner to reach it.