You are the backpack. The galaxy is your problem.

S.A.C.K. drops you as a living backpack onto an abstract moon surface, surrounded by relentless alien monsters. Your only goal: survive, adapt, deploy your defenses, and come back stronger.

Bullet Heaven Meets Tower Defense

Battle through massive swarms with a fully customizable arsenal of up to 6 unique weapons and towers in your inventory.

• Your Tower Limit: You have a max limit for active towers on the map. Upgrade this limit as you level up!

• Total Deployment Freedom: Use your unlocked towers however you want within your limit. Have 3 different tower types in your inventory? You can place 4 Fire Towers and 2 Baseball Towers, spam 7 Standard Towers, or ignore towers entirely and rely purely on mobile firepower.

Your Base Grows with You

Every 5 levels, you choose a direction to expand the map. Expanding gives you more space to place towers and dodge enemies, but beware: every expansion introduces new obstacles and portals that send fresh alien waves your way.

5 Bosses + 1 High-Score Boss

• Boss Evolutions: Defeat up to 5 unique bosses per run. Every victory rewards you with a choice of powerful evolutions for your weapons and towers, while introducing tougher enemy types to the map.

• The Final High-Score Boss: Survived all 5 bosses? Prepare for the ultimate test. The final boss is an unbeatable menace designed solely to challenge your complete build and push your high score to the absolute limit!

Upgrade Cards & Permanent Progression

• In-Run Upgrades & Rarity Tiers: Level up to draft randomized upgrade cards (featuring common to legendary rarity tiers) to raise your tower placement limit, boost stats, or empower your arsenal.

• Meta-Progression: Collect gold dropped by aliens to buy permanent stat upgrades between runs—increase your damage, range, build limits, and survivability.

Key Features:

• Custom Loadouts: Up to 6 unique weapons and towers in your inventory to mix and match.

• Flexible Tower Deployment: Place your unlocked towers freely on the map up to your current tower limit.

• Expandable Map Grid: Expand your map every 5 levels for more placement space—and new enemy portals.

• Boss Evolutions: Defeat 5 bosses to claim run-changing weapon and tower evolutions.

• Endless High-Score Boss: Challenge an unbeatable final boss to show off your ultimate build.

• Card Drafting & Rarity System: Pick in-run upgrades with distinct rarity tiers.

• Permanent Meta-Progression: Use earned gold between runs for permanent stat boosts.

• Charming Visual Style: A stylized mix of pixel-art effects and clean vector graphics in a family-friendly alien setting.