Upgrades available for purchase:
Increases defense points by 50% for 10 minutes. Costs 1000 credits.
Reduces enemy spawn interval by 50% for 5 minutes. Costs 2000 credits.
Increases shield points by 25% for 15 minutes. Costs 3000 credits.
Boosts defense points by 100% for 1 minute. Costs 5000 credits.