Boss Tower I

Review — Castle Climb 5 min

• Each floor was a 192-letter save code in the levels list. unpack code :: custom cut it into rows, and the Tile sprite stamped them.
• The knight never touched a colour. tile at (x) (y) :: custom read the letter at a point, and four corners kept it out of the bricks.
• Coins and keys vanished by rewriting one letter, a key opened a door, and a checkpoint moved start x and start y.
• Three lives, a “Floor cleared!” moment, and a playing switch: 1 while the knight can move, 0 on the title card and between floors.

Quick-fire

The guards in Castle Climb’s last Extra walked left, then right, then left again, for ever. What is the smallest thing a guard has to remember to know which way to walk next?

Today’s Topic 3 min

• A boss as a state machine: one phase variable
• Four phases: idle, wind-up, attack, recover
• A tell: warning the player before an attack
• A window: the only time the boss can be hurt

Learning outcome

By the end of this lesson you will be able to:

1. Drive a sprite with one variable that names what it is doing.
2. Give an attack a warning a player can see and react to.
3. Let a hit count only in the right phase.

Learnt — a boss in four phases 28 min

1 · A boss is a cycle

A good boss is not random. It does the same four things, in the same order, again and again. Once you learn the order, you can win.

One variable, phase, holds the word for what the dragon is doing now. That is a state machine: a few states, and rules for moving between them.

2 · One variable, one script that asks it

In Storm Flight, a button changed the state. Here nobody presses anything. Each phase does its job, then names the next phase. That is the whole trick.

when flag clicked
go to x: (0) y: (70)
set [phase v] to [idle]
forever
  if <(phase) = [idle]> then
    switch costume to [dragon-a v]
    glide (1.2) secs to x: (pick random (-150) to (150)) y: (70)
    set [phase v] to [wind-up]
  end
  if <(phase) = [wind-up]> then
    switch costume to [dragon-b v]
    set [target x v] to (knight x)
    broadcast [warn v]
    glide (0.3) secs to x: (target x) y: (70)
    wait (0.7) seconds
    set [phase v] to [attack]
  end
  if <(phase) = [attack]> then
    switch costume to [dragon-c v]
    create clone of [Fireball v]
    wait (0.8) seconds
    set [phase v] to [recover]
  end
  if <(phase) = [recover]> then
    switch costume to [dragon-a v]
    glide (0.4) secs to x: (x position) y: (0)
    wait (1.2) seconds
    glide (0.4) secs to x: (x position) y: (70)
    set [phase v] to [idle]
  end
end
            

The knight posts its own x into knight x every turn of its loop. That is how the dragon knows where to aim.

3 · The tell

A person needs a moment to see something and react — a little under half a second. An attack with no warning beats every player, every time. That is not hard. It is unfair.

The wind-up is the tell. The dragon crouches, and a marker blinks on the floor where the fire will land. Both demos below use the same knight: it notices, waits 0.4 seconds like a person would, then steps away.

when I receive [warn v]
go to x: (target x) y: (-150)
repeat (5)
  show
  wait (0.1) seconds
  hide
  wait (0.1) seconds
end
            

4 · The window

The knight throws bolts. A bolt only hurts the dragon in the recover phase. At any other time it bounces off. So the player has to wait for the window.

when I start as a clone
go to x: (knight x) y: (-80)
show
repeat until <(y position) > (170)>
  change y by (10)
  if <touching [Dragon v]?> then
    if <(phase) = [recover]> then
      change [hits v] by (1)
    else
      change [bounced v] by (1)
    end
    delete this clone
  end
end
delete this clone
            

Basic — the first boss fight 25 min

Start a new project. You will build the fight from the Learnt yourself.

Step 1 — the hall and the cast

1. Choose the backdrop Castle 3. Delete the cat.
2. Add the Dragon and the Knight from the library. Set both to size 50.
3. Click the Knight’s direction and choose the left-right arrows, so it never turns upside down.
4. Paint three small sprites: Fireball (a red circle), Bolt (a small blue circle) and Marker (a red ring).

Step 2 — the variables

1. For all sprites: phase, target x, knight x, hits and burns.
2. On the Knight, for this sprite only: next shot.

Step 3 — the knight

when flag clicked
go to x: (-150) y: (-115)
set [hits v] to (0)
set [burns v] to (0)
set [next shot v] to (0)
forever
  if <key (right arrow v) pressed?> then
    point in direction (90)
    change x by (6)
  end
  if <key (left arrow v) pressed?> then
    point in direction (-90)
    change x by (-6)
  end
  set [knight x v] to (x position)
  if <<key (space v) pressed?> and <(timer) > (next shot)>> then
    set [next shot v] to ((timer) + (0.3))
    create clone of [Bolt v]
  end
end
            

Step 4 — the dragon’s brain

Build the four-phase script from Learnt section 2 on the Dragon.

Step 5 — fire, marker, bolt

when flag clicked
hide

when I start as a clone
go to x: (target x) y: (40)
show
repeat until <(y position) < (-150)>
  change y by (-8)
  if <touching [Knight v]?> then
    change [burns v] by (1)
    delete this clone
  end
end
delete this clone
              

when I start as a clone
go to x: (knight x) y: (-80)
show
repeat until <(y position) > (170)>
  change y by (10)
  if <touching [Dragon v]?> then
    if <(phase) = [recover]> then
      change [hits v] by (1)
    end
    delete this clone
  end
end
delete this clone
              

The Marker is the blinking script from Learnt section 3, with a hide on the green flag.

A tell you cannot miss

The marker is on the floor. A player watching the dragon might miss it.

• During the wind-up, the dragon flashes bright four times.
• The wind-up still lasts one second.
• The dragon looks normal again when it attacks.

repeat (4)
  set [brightness v] effect to (60)
  wait (0.1) seconds
  set [brightness v] effect to (0)
  wait (0.075) seconds
end
            

One hit per window

Holding space in the window lands four bolts at once. That makes the fight too easy.

• The first bolt that lands ends the window straight away.
• Later bolts bounce off, even though the dragon is still low.
• The dragon’s own script is not changed at all.

if <(phase) = [recover]> then
  change [hits v] by (1)
  set [phase v] to [idle]
end
            

The dragon gloats

Make getting burnt cost something more than one point.

• If the fireball burns the knight, the dragon says “Ha!” and skips its rest.
• If the fireball misses, the dragon rests as usual.
• So a burn also means no window this time.

if <(phase) = [attack]> then
  switch costume to [dragon-c v]
  set [scorched v] to (0)
  create clone of [Fireball v]
  wait (0.8) seconds
  if <(scorched) = (1)> then
    say [Ha!] for (0.8) seconds
    set [phase v] to [idle]
  else
    set [phase v] to [recover]
  end
end
            

A dodge roll

The knight gets a state machine of its own.

• A roll variable: ready, rolling or tired.
• Press ↓ when ready: the knight rolls for 0.7 seconds and fire cannot hurt it.
• Then it is tired for 0.8 seconds and cannot roll again.
• The knight looks see-through while it rolls.

when [down arrow v] key pressed
if <(roll) = [ready]> then
  set [roll v] to [rolling]
  set [ghost v] effect to (60)
  wait (0.7) seconds
  set [roll v] to [tired]
  set [ghost v] effect to (0)
  wait (0.8) seconds
  set [roll v] to [ready]
end
            

if <<touching [Knight v]?> and <not <(roll) = [rolling]>>> then
  change [burns v] by (1)
  delete this clone
end
            

Summary 5 min

• A boss is a state machine: one phase variable, and one script that asks it.
• Each phase does its job, then names the next one.
• A wind-up warns the player. An attack with no warning is unfair, not hard.
• A hit counts only in the recover window, because the bolt checks the phase.

State machine

A few states, and the rules for moving from one to the next.

Phase

The state a boss is in right now.

Tell

A warning that shows an attack is coming.

Window

The short time when the boss can be hurt.

Hand in

Save your boss fight. Next lesson the dragon stops doing the same attack every time: it reads its moves from a list.