Ascend to ZEROThe Underground Bunker in Ascend to ZERO is your permanent hub between runs, and how you invest your Zero Keys here determines the trajectory of your entire progression. Poor upgrade decisions waste hours of playtime, while optimized investments make every subsequent run dramatically easier. This guide covers the complete upgrade priority system, NPC rescue order, and long-term bunker strategy.
Understanding the Bunker Economy
Zero Keys are the permanent currency earned during each run. Your key earnings depend on how far you progress, how many enemies you kill, and how much timer you have remaining when the run ends. Every Zero Key you spend is a permanent investment — unlike run-specific chips, bunker upgrades persist across all future runs.
Key principle: Concentrate your Zero Key investments on a single avatar first. Spreading keys across multiple avatars before one is fully upgraded dilutes your power and slows progression. Only start investing in a second avatar after your primary avatar has reached at least the 200-key investment milestone.
| Investment Phase | Key Budget | Focus | Expected Run Impact |
|---|---|---|---|
| Early (0-100 keys) | First 5-10 runs | Starting Time + XP Gain | 30s→45s starting timer |
| Mid (100-300 keys) | Runs 10-30 | Damage + Defense + 2nd XP | Consistent Stage 2 clears |
| Late (300-600 keys) | Runs 30-80 | All remaining stats | Regular Stage 3-4 runs |
| Endgame (600+ keys) | Run 80+ | Secondary avatar | Versatile run options |
Time Machine Upgrade Priority
The Time Machine is the most important bunker facility. It converts Zero Keys into permanent stat upgrades that apply to your avatar at the start of every run. The order in which you purchase these upgrades matters enormously because of the compounding effect — early upgrades multiply the value of later ones.
Tier 1 — Buy Immediately (0-75 keys):
| Upgrade | Cost | Effect | Why First |
|---|---|---|---|
| Starting Time +5s | 50 keys | Run begins with 35s instead of 30s | Extra buffer prevents early deaths |
| XP Gain +10% | 75 keys | Level 10% faster each run | Faster levels = earlier power spikes |
These two upgrades are universally agreed upon by the community as the highest priority. Starting Time +5s gives you a 17% larger safety window at the start of every run, which is the most dangerous phase. XP Gain +10% accelerates your entire run because higher levels mean more damage, which means faster kills, which means more timer extension.
Tier 2 — Buy Next (75-250 keys):
| Upgrade | Cost | Effect | Why Second |
|---|---|---|---|
| Starting Time +5s (2nd) | 125 keys | Run begins with 40s | Compounds with first upgrade |
| Avatar Damage +5% | 100 keys | All attacks deal 5% more | Scales exponentially with inflation |
| XP Gain +10% (2nd) | 150 keys | Level 20% faster total | Stacking XP is extremely powerful |
At this tier, you are building on the foundation of Tier 1. The second Starting Time upgrade brings you to 40 seconds — enough buffer to survive most early-game mistakes. Damage +5% might seem small, but because Ascend to ZERO uses an inflation growth model where stats scale by orders of magnitude, a 5% permanent bonus compounds to enormous numbers in late runs.
Tier 3 — Buy When Available (250-500 keys):
| Upgrade | Cost | Effect | Why Third |
|---|---|---|---|
| Defense +5% | 100 keys | Take 5% less time damage per hit | Reduces timer loss from mistakes |
| Time Recovery +10% | 150 keys | Kill rewards give 10% more seconds | Extends positive feedback loop |
| Avatar Damage +5% (2nd) | 200 keys | 10% total permanent damage | Excellent late-run scaling |
Defense is less critical early because the best defense in Ascend to ZERO is not getting hit — which time freeze enables. However, once you have solid fundamentals, Defense +5% provides a safety net for imperfect play. Time Recovery +10% is excellent because it directly amplifies the core gameplay loop of killing enemies to extend your timer.
NPC Rescue Order
NPCs rescued during runs unlock new bunker facilities. Each NPC provides a unique service, and the order in which you encounter them during runs is somewhat random. However, you can influence which NPCs you encounter by choosing which rooms to explore.
Priority rescue order:
| Priority | NPC | Service | Why Priority |
|---|---|---|---|
| 1 | Engineer | Time Machine upgrades | Core progression system |
| 2 | Quartermaster | Equipment shop | Access to better starting gear |
| 3 | Scientist | Chip analysis | See chip stats before pickup |
| 4 | Medic | Extra resurrection | One free revive per run |
| 5 | Scout | Stage map data | See room layouts before entering |
Engineer first because without the Time Machine, no other upgrade matters. The Quartermaster lets you purchase permanent equipment that carries between runs, providing a significant early-game power boost. The Scientist's chip analysis service is valuable for experienced players who want to optimize chip selection, but less critical for beginners still learning the game.
Equipment Salvaging Strategy
Equipment found during runs can be salvaged at the bunker for permanent stat bonuses. This creates a secondary progression system parallel to Zero Key investments.
Salvaging rules:
- Common equipment: Salvage for small permanent stat boosts
- Uncommon equipment: Salvage for moderate boosts or dismantle for crafting materials
- Rare equipment: Equip rather than salvage — the active benefit exceeds the salvage value
- Epic/Legendary: Always equip — these items are rare and powerful
The salvaging trap: Do not salvage rare or better equipment early in your progression. A rare weapon equipped during runs provides more value than the small permanent stat boost from salvaging it. Only salvage rare equipment once you have found a replacement that is equally good or better. For detailed equipment guidance, see our Equipment Salvaging Guide.
Long-Term Bunker Strategy
After your first 50+ runs, bunker optimization shifts from "what to buy next" to "how to maximize efficiency across all systems." The key insight is that bunker systems are interconnected — Time Machine upgrades make runs easier, which earns more Zero Keys, which funds more upgrades, in a positive feedback loop.
Efficiency milestones:
| Milestone | When | Impact |
|---|---|---|
| 40s starting timer | ~15 runs | Can comfortably explore Stage 1 |
| 20% XP boost | ~25 runs | Level scaling feels noticeably faster |
| Full Tier 2 upgrades | ~35 runs | Consistent Stage 2 clears |
| Full Tier 3 upgrades | ~60 runs | Stage 3 becomes accessible |
| 2nd avatar fully upgraded | ~100+ runs | Flexible run strategies |
The most important thing is consistency. Every run, even short ones, earns Zero Keys. Players who stop running after a few failed attempts slow their progression dramatically. Even a 2-minute run that earns 5-10 keys is better than no run at all.
Bunker Upgrade Priority for Different Avatar Builds
Different avatars benefit from different Time Machine upgrade priorities. While starting time is universally valuable, subsequent upgrades should match your avatar's strengths and weaknesses.
Blossom Blade priority:
- Starting Time +5s (x2) — Glass cannon needs buffer
- Starting Damage +10% — Accelerates sword chip damage foundation
- XP Gain +10% (x2) — Faster leveling compounds into more sword damage
- Time Recovery +5% — Partially offsets glass cannon timer losses
- Defense +% — Last resort; low priority since freeze prevents most damage
Frost Moon Hacker priority:
- Starting Time +5s (x2) — Survives weak early game
- XP Gain +10% (x2) — Compounds drone scaling through levels
- Time Recovery +5% (x2) — Drone kills return more timer
- Starting Damage +10% — Boosts base drone damage slightly
- Defense +% — Moderate value since Hacker stays at range from drones
Golden Gunslinger priority:
- Starting Time +5s (x2) — Standard buffer
- Starting Damage +10% — Ranged DPS acceleration
- XP Gain +10% (x2) — Standard compounding
- Time Recovery +5% — Ranged kills extend timer efficiently
- Defense +% — Very low priority; gunslinger rarely takes hits
Chrono Child priority:
- Starting Time +5s (x2) — Standard buffer
- Starting Damage +10% — AOE damage scales with base
- XP Gain +10% (x2) — Standard compounding
- Time Recovery +5% — AOE hits multiple enemies per kill for timer gain
- Defense +% — Moderate value; AOE center positioning invites some hits
For the full upgrade cost and path, see our Bunker Complete Upgrade Path.
Frequently Asked Questions
Can I reset bunker upgrades and get my Zero Keys back?
No. All Zero Key purchases at the Time Machine are permanent and cannot be refunded. This is why the priority order matters — spending 100 keys on a low-priority upgrade early means delaying a high-priority one. Follow the tier system above to avoid wasted investments.
Should I invest in my second avatar before my first is fully upgraded?
Generally, no. The compounding nature of Time Machine upgrades means that a fully upgraded primary avatar dramatically outperforms two partially upgraded avatars. Only invest in a second avatar when your primary has reached the 200+ key milestone and you are consistently clearing Stage 2 or beyond.
What is the fastest way to earn Zero Keys?
Longer runs that reach deeper stages earn significantly more keys. A run that reaches Stage 3 typically earns 30-50 keys, while a Stage 1-only run might earn 10-15. Investing in Starting Time and XP Gain first creates a positive loop where you survive longer and earn more keys per run. See our Progression Guide for detailed key earning rates.
Where can I find community upgrade priorities?
The official Discord has pinned guides in the bunker-upgrades channel with community-voted priority lists. The Steam community hub also hosts discussion threads on optimal upgrade paths.