SIEGE SURGE: Tower Panic

1. WAVE HITS: A formation of goblin-soldiers marches down the 480px-wide corridor at a base speed of 60px/sec. If the front unit crosses the red baseline at the bottom of the screen within 2 seconds of wave start without being fired upon, the Panic Multiplier increments by 0.2x immediately — not at end-of-wave.
2. COLLECT SCRAP: Each destroyed goblin ejects 1–3 spinning square scrap tokens that drift 40–80px from the kill point in a random arc. Each token has a 3-second lifetime shown by a shrinking white border ring. Tap the token inside that window: +1 scrap banked. Miss it: that scrap is gone, the bank does not compensate, and the Panic Multiplier ticks +0.05x per missed token.
3. UPGRADE TOWER: Between waves (a fixed 6-second window), spend banked scrap on one of three randomly drawn upgrades displayed as labelled icon cards — Chain Shot (cost: 4 scrap, fires a ricochet projectile hitting up to 3 targets per shot), Cluster Burst (cost: 6 scrap, every 5th shot detonates into 8 shrapnel fragments), or Overdrive Pulse (cost: 9 scrap, increases fire rate by 40% for the next two waves then drops 10% below baseline permanently). Selecting any card triggers a Surge Event: a bonus wave of goblins at 2× the current wave's spawn count spawns exactly 5 seconds after the card is tapped, regardless of whether the 6-second upgrade window has closed.
4. SURVIVE THE SURGE: During the Surge wave, each consecutive kill without a miss increments the on-screen Combo Counter (white numeral, top-right, 36pt). Reaching a 10-kill streak during a single Surge spawns a Contraband Crate — a spinning octagon that lands at a fixed point in the corridor centre. Tapping it within 4 seconds opens it: the lid animates open in 12 frames, the UI accent colour flashes for 0.4 seconds, and a new permanent enemy mutation is named in 48pt bold (e.g. 'ROCKET GOBLINS UNLOCKED — all future waves include 15% Rocket Goblin substitution'). If the Crate is not tapped within 4 seconds, it detonates, adding +0.3x to the Panic Multiplier and granting no mutation.

• Hesitation is measured: every 2 consecutive seconds in which the player neither taps a scrap token nor fires a manual ability, the Panic Multiplier increments +0.1x. This counter never resets for any reason during a run.
• Every upgrade card selection immediately schedules a Surge Event at T+5 seconds — there is no upgrade action that does not trigger a Surge.
• The Combo Counter resets to zero if any goblin reaches the red baseline or if 1.5 seconds pass between kills. A 10-kill unbroken streak during a Surge is the only trigger for a Contraband Crate drop.
• Scrap tokens display a shrinking white border ring as their 3-second timer. When the ring closes completely the token plays a 4-frame grey flash and disappears. Missed tokens add +0.05x to the Panic Multiplier each.
• If 3 or more goblin units cross the red baseline in a single wave (not a Surge), the Panic Multiplier doubles its current value instantly — this is shown by the counter flashing red for 0.8 seconds and emitting a low-frequency pulse sound.

• Unlock new tower chassis by surviving to wave 15 with a Panic Multiplier reading 1.5x or below at the moment wave 15 ends — the chassis unlocked is displayed as a vector schematic with its base fire pattern diagrammed (e.g. 'Scatter Chassis: fires 5 projectiles in a 30-degree arc, base damage 60% of Standard Chassis per projectile').
• Every Contraband Crate opened adds a named entry to the permanent Chaos Codex: a grid of 32 silhouette cards, each showing the mutation name, the wave it was first unlocked, and the run number. Filling all 32 cards requires opening Crates across multiple runs because only 4–6 mutations are available per run seed.
• Prestige ('Go Rogue'): at run-end, the player may bank one installed upgrade into the Rogue Slot — next run starts with that upgrade already installed at level 1 and its first Surge pre-scheduled to trigger at the 8-second mark of wave 1, before the player has collected any scrap.
• Weekly Siege Dossiers: a fixed 10-wave sequence with a locked enemy roster published every Monday, expiring Sunday. The leaderboard ranks by highest Combo Counter peak (not score), and the top 500 players receive an exclusive tower skin — a named vector illustration (e.g. 'Week 12: The Brutalist Obelisk') added permanently to their chassis roster.

• STEP 1 — CORE LOOP VERTICAL SLICE (Days 1–2): Build a single Unity scene: one static rectangle as the tower, one corridor lane, goblins as 24×24px coloured squares moving at 60px/sec, scrap tokens as 12×12px white squares with a 3-second shrinking-border shader, and three hardcoded upgrade buttons (no random draw yet). Wire the Surge spawn: tapping any upgrade button sets a 5-second timer that doubles the next spawn count. Target: a playable loop where the upgrade→Surge→collect→upgrade rhythm can be felt within 10 minutes, with no art assets required.
• STEP 2 — PANIC MULTIPLIER TUNING (Days 3–4): Instrument the Panic Multiplier with all five increment triggers (2-second hesitation, missed token, baseline breach ×1, baseline breach ×3 doubles, upgrade-window expiry). Run 20 timed sessions with three internal testers, logging the wave number at death and the Panic Multiplier value at death. Target: median death at wave 9–11 for first-time players. If median death is below wave 7, reduce the hesitation tick from +0.1x to +0.07x. If median death is above wave 13, reduce the breach-doubling threshold from 3 units to 2 units.
• STEP 3 — CONTRABAND CRATE + CHAOS CODEX (Day 5): Implement 8 hardcoded Crate mutations as spawn-table modifiers (e.g. Rocket Goblin: +15% substitution rate, moves at 1.3× base speed, requires 2 hits to kill). Wire the Combo Counter reset conditions. Build the Codex as a 32-slot grid with placeholder silhouettes. Capture a 15-second screen recording of a wave-12 Surge chain where a Crate drops and a new enemy type appears — use this recording as the primary ad creative test asset before committing to full production art.

• UPGRADE LOOP: The upgrade card is both the reward delivery mechanism and the difficulty escalation trigger — selecting it starts a 5-second countdown to a harder wave, making the upgrade action a commitment with an immediate, named consequence rather than a safe power gain.
• COLLECTION LOOP: Scrap tokens have a shrinking white border ring as a literal countdown display — the collect action is time-pressured by a visible per-token timer, and missing a token has two named consequences: permanent scrap loss and a +0.05x Panic Multiplier tick.
• RARE ITEM REVEAL: Contraband Crates require a 10-kill unbroken streak during a Surge wave — a condition that depends on tower damage (prior upgrades) and scrap collection (prior collection behaviour), making the Crate a product of two prior skill loops rather than a random drop.
• COLLECT INTERACTION: Scrap tokens drift 40–80px from kill points on an otherwise empty off-white field — the collect verb requires physical movement of the tap across the screen, not a stationary press, making it a distinct motor action from the fire and upgrade taps.
• DEFEND INTERACTION: The red baseline is a literal loss condition line — goblins crossing it increment the Panic Multiplier, and 3 crossings in one wave double it. Defence is measured in units-crossed, not health points, making the failure signal a count rather than a bar.
• SATISFYING CHAIN: The upgrade→Surge→combo→Crate sequence has four named events across approximately 12 seconds: card tap, 5-second countdown, 10th kill, Crate open. Each event has a distinct audio-visual signal (countdown number, combo numeral increment, octagon drop, lid animation + colour flash), making the chain a sequence of discrete beats rather than a continuous blur.
• TOWER TOPOLOGY: The single corridor with a fixed red baseline and a fixed tower position means all player attention is on the 480×320px play area — no camera movement, no placement decisions, no UI panels overlapping the field during a wave. Every pixel of the play area is used for enemy positions, scrap tokens, or the Combo Counter.