There is actually a loose Mechanic. U might had an Bug, usually when a Warrior is Dead on a Lane the Damage goes on the Castle if its not Defended. Once the Castle Defense drops on Zero or below the Game should End.
I just saw that the gold and the rounds are swapped in the Game Over Panel. But the Game Over should normally happen when you can't defend a lane anymore. I'm sorry you experienced a bug, I'll definitely take a look to see what could be causing it.
Your damage is irrelevant, you just need enough life and, or shield to survive an enemy's attack. Basically you just block an attack and either the hero survives or dies. But thanks for pointing out that it's unclear. I'd like to revise some parts of the UI and the playfield to make things clearer.