Can't Afford It

Lesson 12 of 22 in Coddy's Roblox Game: Tower Defense course.

So far towers are free, and the server builds whatever it is asked. A player controls their own device, so an exploiter can fire Remotes.Place with anything: a tower on top of another, on the baseplate, a tower that does not exist, with no money at all.

So the server checks every request before it builds, and returns at the first thing that is wrong:

the pad is an Instance in workspace.Pads and has no tower yet (an attribute Used that placeTower sets),
the kind is a string naming a real tower,
the player has the money; then the server takes it.

if typeof(pad) ~= "Instance" or pad.Parent ~= workspace.Pads then
	return
end

The client asks, the server decides. The checks in this lesson play the exploiter.

Challenge

Medium

Make towers cost money, and refuse bad requests.

In the Place handler, return unless the pad is an Instance in workspace.Pads without the Used attribute.
Return unless the kind is a string naming a tower in Settings.Towers.
Return when Money is below its Price; else take it and build.
placeTower sets the pad's Used to true.

Press Play and buy until Money runs low: the next tower is refused. Then Check.

Try it yourself

local Players = game:GetService("Players")
local ServerStorage = game:GetService("ServerStorage")
local Debris = game:GetService("Debris")
local Settings = require(game.ReplicatedStorage.Settings)

local remotes = game.ReplicatedStorage.Remotes
local enemies = workspace.Enemies
local towers = workspace.Towers

print("The base can take " .. Settings.BaseHealth .. " hits")

-- The waypoints, in order (they are named 1, 2, 3...)
local waypoints = workspace.Path.Waypoints:GetChildren()
table.sort(waypoints, function(a, b)
	return tonumber(a.Name) < tonumber(b.Name)
end)

-- The base's health, as an attribute of the workspace: every client can read it
workspace:SetAttribute("BaseHealth", Settings.BaseHealth)

-- Every player gets Money on the leaderboard
Players.PlayerAdded:Connect(function(player)
	local leaderstats = Instance.new("Folder")
	leaderstats.Name = "leaderstats"
	leaderstats.Parent = player

	local money = Instance.new("IntValue")
	money.Name = "Money"
	money.Value = Settings.StartMoney
	money.Parent = leaderstats
end)

-- An enemy reached the base
local function damageBase(amount)
	local health = math.max(0, workspace:GetAttribute("BaseHealth") - amount)
	workspace:SetAttribute("BaseHealth", health)
	print("Base: " .. health)
end

-- Walk an enemy along the path. MoveTo gives up after 8 seconds and says so
-- (MoveToFinished with false), so a slow enemy is sent again until it arrives.
local function walkPath(enemy)
	local humanoid = enemy.Humanoid
	for i = 2, #waypoints do
		repeat
			humanoid:MoveTo(waypoints[i].Position)
		until humanoid.MoveToFinished:Wait() or not enemy.Parent
		if not enemy.Parent then
			return
		end
	end
	-- it reached the base
	damageBase(Settings.Enemies[enemy.Name].Damage)
	enemy:Destroy()
end

-- A new enemy at the gate, walking the path
local function spawnEnemy(kind)
	local enemy = ServerStorage.Enemies[kind]:Clone()
	-- dropped in just above the first waypoint
	enemy:PivotTo(waypoints[1].CFrame + Vector3.new(0, 5, 0))
	local info = Settings.Enemies[kind]
	enemy.Humanoid.MaxHealth = info.Health
	enemy.Humanoid.Health = info.Health
	enemy.Humanoid.WalkSpeed = info.Speed
	enemy.Parent = enemies
	enemy.Humanoid.Died:Connect(function()
		-- every player earns the reward
		for _, player in Players:GetPlayers() do
			player.leaderstats.Money.Value += info.Reward
		end
		enemy:Destroy()
	end)
	task.spawn(walkPath, enemy)
	return enemy
end

-- The nearest living enemy within range of a point, or nil
local function nearestEnemy(position, range)
	local best, bestDistance = nil, range
	for _, enemy in enemies:GetChildren() do
		local root = enemy:FindFirstChild("HumanoidRootPart")
		if root and enemy.Humanoid.Health > 0 then
			local distance = (root.Position - position).Magnitude
			if distance <= bestDistance then
				best, bestDistance = enemy, distance
			end
		end
	end
	return best
end

-- A flash of light from a tower to its target
local function beam(from, to)
	local ray = Instance.new("Part")
	ray.Anchored = true
	ray.CanCollide = false
	ray.CanTouch = false
	ray.Material = Enum.Material.Neon
	ray.Color = Color3.fromRGB(255, 240, 150)
	ray.Size = Vector3.new(0.2, 0.2, (to - from).Magnitude)
	ray.CFrame = CFrame.lookAt((from + to) / 2, to)
	ray.Parent = workspace
	Debris:AddItem(ray, 0.1)
end

-- A shot: damage the target
local function shoot(tower, target, info)
	local damage = info.Damage
	local at = target.HumanoidRootPart.Position
	target.Humanoid:TakeDamage(damage)
	beam(tower.Head.Position, at)
end

-- A tower's life: aim at the nearest enemy in range, shoot, wait its cooldown
local function runTower(tower)
	local info = Settings.Towers[tower.Name]
	while tower.Parent do
		local target = nearestEnemy(tower.Head.Position, info.Range)
		if target then
			tower.Head.CFrame = CFrame.lookAt(tower.Head.Position, target.HumanoidRootPart.Position)
			shoot(tower, target, info)
			task.wait(info.Cooldown)
		else
			task.wait(0.1)
		end
	end
end

-- Build a tower on a pad
local function placeTower(pad, kind)
	local tower = ServerStorage.Towers[kind]:Clone()
	tower:PivotTo(pad.CFrame + Vector3.new(0, pad.Size.Y / 2 + 1, 0))
	tower.Parent = towers
	task.spawn(runTower, tower)
end

-- The client asks for a tower on a pad
remotes.Place.OnServerEvent:Connect(function(player, pad, kind)
	-- Lesson 12: Can't Afford It goes here
	placeTower(pad, kind)
end)

-- One wave: its enemies one by one, then wait until they are all gone
local function runWave(list)
	for _, kind in list do
		spawnEnemy(kind)
		task.wait(Settings.SpawnGap)
	end
	while #enemies:GetChildren() > 0 do
		task.wait(0.2)
	end
end

-- The waves, each after a break
for i, list in Settings.Waves do
	task.wait(Settings.WaveBreak)
	runWave(list)
end