diff --git a/spec/deposit_spec.cr b/spec/deposit_spec.cr
new file mode 100644
index 0000000..83ba11c
--- /dev/null
+++ b/spec/deposit_spec.cr
@@ -0,0 +1,38 @@
+require "./spec_helper"
+
+module Game::Test
+  describe Deposit do
+    it "should be created fulfilled" do
+      dep = Deposit.new(Resources::Type::Crystals, 100)
+      dep.cap.should eq 100
+      dep.cur.should eq 100
+    end
+
+    it "can be created partially filled" do
+      dep = Deposit.new(Resources::Type::Crystals, 100, 20)
+      dep.cap.should eq 100
+      dep.cur.should eq 20
+    end
+
+    it "should be decreased with span" do
+      dep = Deposit.new(Resources::Type::Crystals, 100)
+      dep.dec Deposit::Span.new(Resources::Type::Crystals, 20)
+      dep.cap.should eq 100
+      dep.cur.should eq 80
+    end
+
+    it "should not be increased above capacity" do
+      dep = Deposit.new(Resources::Type::Crystals, 100, 20)
+      dep.inc Deposit::Span.new(Resources::Type::Crystals, 100)
+      dep.cap.should eq 100
+      dep.cur.should eq 100
+    end
+
+    it "should not be decreased below zero" do
+      dep = Deposit.new(Resources::Type::Crystals, 100)
+      dep.dec Deposit::Span.new(Resources::Type::Crystals, 120)
+      dep.cap.should eq 100
+      dep.cur.should eq 0
+    end
+  end
+end
diff --git a/src/game/deposit.cr b/src/game/deposit.cr
new file mode 100644
index 0000000..96c42ff
--- /dev/null
+++ b/src/game/deposit.cr
@@ -0,0 +1,40 @@
+module Game
+  class Deposit
+    class Span
+      def initialize(@res : Resources::Type, @cap : Capacity)
+      end
+
+      getter res
+      getter cap
+    end
+
+    @cur : Capacity = 0
+
+    def initialize(@res : Resources::Type, @cap : Capacity)
+      @cur = @cap
+    end
+
+    def initialize(@res : Resources::Type, @cap : Capacity, @cur : Capacity)
+    end
+
+    getter res
+    getter cap
+    getter cur
+
+    def inc(span : Span)
+      check_res span.res
+      @cur = Math.min(@cap, @cur + span.cap)
+    end
+
+    def dec(span : Span)
+      check_res span.res
+      @cur = Math.max(0, @cur - span.cap)
+    end
+
+    private def check_res(other_res : Resources::Type)
+      if @res != other_res
+        raise ResourceMismatch.new
+      end
+    end
+  end
+end