Your bullet climbs at 480 pixels per second. The bottom alien row sits 600 pixels above your firing point at level 1, so your shot needs 1.25 seconds to reach it. The top row sits 760 pixels up and needs 1.58 seconds. Those two flight times set every firing decision in the game, because you own only two bullets at a time.
The three speeds in this Alien Defense build
You slide at 260 pixels per second. Your bullet climbs at 480. Alien bombs fall at 200, slower than your shot climbs. A bomb released from the top row at y 120 falls 780 pixels to your line and takes 3.9 seconds to arrive.
The gap is the core fact. Anything you need to dodge, you outrun horizontally on paper. Anything you shoot at, you reach more than twice as fast as it reaches you.
The two bullet cap and the 0.3 second cooldown
The engine refuses a third shot while two of yours are airborne and adds a 0.3 second cooldown after every shot. Holding fire auto-shoots at the cooldown rate, about 3.3 shots per second, but the airborne cap is the real constraint.
A shot that misses and flies over the swarm stays airborne for its full flight time, up to 1.58 seconds at level 1. During that window you have one bullet left. Fire the first shot, confirm it will land, and hold the second for a guaranteed alignment instead of spraying both into empty columns.
Lead the swarm, because it steps while you shoot
At level 1 the swarm steps 18 pixels sideways every 0.6 seconds. Your bullet spends up to 1.58 seconds in the air, and the swarm takes two or three steps in that time, up to 54 pixels. Aim half an alien width ahead of a moving edge column.
The swarm drops 16 pixels and reverses whenever its next step would pass x 20 or x 520. When the swarm hugs a wall, expect the drop during your flight time and aim one row lower than your target.
Fewer aliens move faster, on a coded curve
Every kill recalculates the swarm. The speed multiplier is 1 plus the dead fraction times 3.5, and the step interval shrinks with the alive ratio down to a floor of 0.05 seconds.
With all 55 alive at level 1 the swarm steps 18 pixels per 0.6 seconds. With one alien alive the multiplier reaches 4.44, speed rises to about 133 pixels per second, and it steps roughly every 0.05 seconds. The last alien jitters. Stop tracking it visually and start cutting it off, because your bullet still needs more than a second to arrive.
Where alien shots come from
Only the bottom-most alive alien of each column can fire, and the engine picks one shooter uniformly at random from those candidates. An empty column contributes nothing. The bomb timer is 1.5 seconds at level 1 with all 55 alive, tightens to about 1.0 second when one alien remains, and never drops below 0.3 seconds.
Clearing a full column removes one firing position. Clearing a bottom row of eleven removes eleven shooters at once, which is the fastest way to thin the bombardment. A full wave pays 990 points, 11 aliens at 30 on top, 22 at 20, 22 at 10.
Shields, the UFO, and the instant loss line
Shields sit at y 760. An impact destroys cells within 5 pixels of contact, each with a 70 percent chance, so shields erode in ragged bites rather than clean holes. Aliens that reach a shield erase cells deterministically within 16 by 14 pixels of contact.
The UFO appears 8 to 18 seconds into a level, then every 12 to 25 seconds after each crossing. It flies at 100 pixels per second at y 60, which gives you about 6 seconds of window. Its value is drawn uniformly from 50, 100, 150, 200, and 300, so the expected payout is 150 for one bullet.
Any alien that reaches y 880 ends the game at once, whatever your remaining lives. Check the swarm altitude before you commit to a UFO chase. After a death you get 2 seconds of invincibility and respawn at center, which is enough time to relocate but not enough to clear a wave.
Honest downsides of fire discipline advice
The shooter pick is a uniform random draw each cycle, so parking under a quiet column protects you from nothing. UFO value is random, so the identical chase pays 50 one game and 300 the next. Late levels also start the swarm 16 pixels lower per level and remove 0.04 seconds from the move interval per level, so both movement and firing compress well before your aim degrades. Discipline raises your floor. It does not beat the timer curve on its own.
Fire discipline checklist
- [ ] Two bullets tracked, the second held for a confirmed line.
- [ ] Aim point half an alien ahead of a moving column.
- [ ] Swarm at a wall edge, aim one row lower for the 16 pixel drop.
- [ ] UFO window timed from its entry side, swarm still above the shields.
- [ ] Bottom rows cleared before the swarm descends past y 760.
Play the [Alien Defense game](/en/space-invaders) and test the flight times first, one shot at a time against the bottom row. If your measured travel disagrees with the 1.25 second figure, or you find a firing pattern that beats the two bullet cap, send the level number and the sequence. Cap exploits are the most useful report you can file.
