diff --git a/src/bullet.lua b/src/bullet.lua
index b8cfc51..a1bb283 100644
--- a/src/bullet.lua
+++ b/src/bullet.lua
@@ -21,10 +21,12 @@ local POWER_DAMAGE_LOOKUP <const> = {
    [4] = 10,
 }
 
-function Bullet:init(power, friendly)
+-- Bullet's defaults assume a friendly bullet
+function Bullet:init(power, vector, collisionMask)
    Bullet.super.init(self)
    self.power = power
    local size = POWER_SIZE_LOOKUP[power]
+   local mask = collisionMask or 0x4
 
    local img = gfx.image.new(size, size)
    gfx.pushContext(img)
@@ -35,20 +37,14 @@ function Bullet:init(power, friendly)
    self:setCollideRect(0, 0, self:getSize())
    self.collisionResponse = gfx.sprite.kCollisionTypeOverlap
 
-   local friendly = friendly or true
-   if friendly then
-      self:setGroupMask(0x4)
-      self:setCollidesWithGroupsMask(0x8)
-      self.direction = 1
-   else
-      self:setGroupMask(0x10)
-      self:setCollidesWithGroupsMask(0x2)
-      self.direction = -1
-   end
+   self:setGroupMask(0x10)
+   self:setCollidesWithGroupsMask(mask)
+
+   self.vector = vector or {x=3, y=0}
 end
 
 function Bullet:update()
-   local collisions = select(3, self:moveWithCollisions(self.x+self.direction, self.y))
+   local collisions = select(3, self:moveWithCollisions(self.x+self.vector.x, self.y+self.vector.y))
    for i=1, #collisions, 1 do
       -- anything the bullet can collide with should be damaged by it
       local obj = collisions[i].other
diff --git a/src/enemy/ebi.lua b/src/enemy/ebi.lua
index dc92ba1..c09f2f4 100644
--- a/src/enemy/ebi.lua
+++ b/src/enemy/ebi.lua
@@ -11,4 +11,6 @@ class("Ebi").extends(Entity)
 function Ebi:init()
    local img = gfx.image.new(10, 10, gfx.kColorBlack)
    Ebi.super.init(self, img, 5)
+
+   self:setCollidesWithGroupsMask(0x3)
 end
diff --git a/src/entity.lua b/src/entity.lua
index 6c701d2..d53cfe5 100644
--- a/src/entity.lua
+++ b/src/entity.lua
@@ -15,9 +15,9 @@ function Entity:init(img, health, armor)
    self:setCollideRect(0, 0, self:getSize())
 
    -- most entities will be enemies, so we configure this mask by default
-   -- We don't set a collider mask because collision is always handled by
-   -- other objects (todo: consider player staying perfectly still)
-   self:setGroupMask(0x8)
+   -- We don't set a collider mask because collision is a bit too variable
+   -- (but we should always include 0x2 and handle player collisions)
+   self:setGroupMask(0x4)
 end
 
 function Entity:damage(amount)
diff --git a/src/kani.lua b/src/kani.lua
index e00fa06..c00e5aa 100644
--- a/src/kani.lua
+++ b/src/kani.lua
@@ -28,7 +28,7 @@ function Kani:init(ui)
 
    self.vector = {x=0,y=0} -- movement direction
    self:setGroupMask(0x2)
-   self:setCollidesWithGroupsMask(0x19)
+   self:setCollidesWithGroupsMask(0xd)
 
    self.reserveCharge = 100
 
diff --git a/src/main.lua b/src/main.lua
index 9ca2a32..49c937f 100644
--- a/src/main.lua
+++ b/src/main.lua
@@ -1,9 +1,9 @@
 -- Collision groups:
--- 0x1 - walls
+-- 0x1 - top and bottom walls
 -- 0x2 - player
--- 0x4 - player bullets
--- 0x8 - enemies
--- 0x10 - enemy bullets
+-- 0x4 - enemies
+-- 0x8 - side walls
+-- 0x10 - bullets
 import "CoreLibs/object"
 import "CoreLibs/graphics"
 import "CoreLibs/sprites"
@@ -30,15 +30,15 @@ function setup()
    enemy:add()
 
    enemy = Ebi()
-   enemy:moveTo(300, 50)
+   enemy:moveTo(270, 50)
    enemy:add()
 
    enemy = Ebi()
-   enemy:moveTo(300, 100)
+   enemy:moveTo(280, 100)
    enemy:add()
 
    enemy = Ebi()
-   enemy:moveTo(300, 150)
+   enemy:moveTo(290, 150)
    enemy:add()
 
    enemy = Ebi()
@@ -51,18 +51,18 @@ function setup()
 end
 
 function makeWalls()
-   makeWall(200, 0, 400, 1)
-   makeWall(0, 120, 1, 240)
-   makeWall(200, 240, 400, 1)
-   makeWall(400, 120, 1, 240)
+   makeWall(200, 0, 400, 1, 0x1)
+   makeWall(0, 120, 1, 240, 0x8)
+   makeWall(200, 240, 400, 1, 0x1)
+   makeWall(400, 120, 1, 240, 0x8)
 end
 
-function makeWall(x, y, w, h)
+function makeWall(x, y, w, h, mask)
    local wall = gfx.sprite.new()
    wall:setSize(w, h)
    wall:setCollideRect(0, 0, wall:getSize())
    wall:moveTo(x, y)
-   wall:setGroupMask(0x1)
+   wall:setGroupMask(mask)
    wall:add()
 end