How do skills work?

Both of these skills can be used during combat with a cooldown:

• Thunder Bolt deals a set amount of damage based on the skill level (starting at 20, +7 damage per level) with a cooldown of 20 seconds.
• Invulnerability makes you immune to all damage for a set amount of time based on the skill level (starting at 3 seconds, +3 seconds per level) with a cooldown of 30 seconds.

You can upgrade your skill after choosing it for a gold cost. Each upgrade costs 500*skillLevel*skillLevel+500 gold bars, with no ceiling on the skill level.