diff --git a/source/TextBlade.Core.Tests/Characters/MonsterTests.cs b/source/TextBlade.Core.Tests/Characters/MonsterTests.cs
new file mode 100644
index 0000000..fd5281e
--- /dev/null
+++ b/source/TextBlade.Core.Tests/Characters/MonsterTests.cs
@@ -0,0 +1,110 @@
+using NUnit.Framework;
+using TextBlade.Core.Characters;
+
+namespace TextBlade.Core.Tests.Characters;
+
+[TestFixture]
+public class MonsterTests
+{
+    [Test]
+    public void Constructor_SetsHealthToTotalHealth()
+    {
+        // Arrange/Act
+        var monster = new Monster("Slime", 10, 5, 3);
+        // Assert
+        Assert.That(monster.CurrentHealth, Is.EqualTo(10));
+    }
+
+    [Test]
+    public void Damage_ReducesHp()
+    {
+        // Arrange
+        var monster = new Monster("Blue Slime", 100, 10, 1);
+
+        // Act
+        monster.Damage(55);
+
+        // Assert
+        Assert.That(monster.CurrentHealth, Is.EqualTo(45));
+    }
+
+    [Test]
+    public void Damage_BottomsOutHealthAtZero()
+    {
+        // Arrange
+        var monster = new Monster("Yellow Slime", 25, 5, 1);
+    
+        // Act
+        monster.Damage(99999);
+
+        // Assert
+        Assert.That(monster.CurrentHealth, Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Attack_DamagesByStrengthMinusTargetToughness()
+    {
+        // Arrange
+        var monster = new Monster("Green Slime", 100, 10, 5);
+        var target = new Character("Asad", 100, 100) { Toughness = 7 };
+        var expectedDamage = monster.Strength - target.Toughness;
+
+        // Act
+        monster.Attack(target);
+
+        // Assert
+        Assert.That(target.CurrentHealth, Is.EqualTo(target.TotalHealth - expectedDamage));
+    }
+    
+    [Test]
+    public void Attack_DamagesByLessThanStrengthMinusTargetToughness_IfTargetIsBlocking()
+    {
+        // Arrange
+        var monster = new Monster("Green Slime", 100, 10, 5);
+        var target = new Character("Asad", 100, 100) { Toughness = 3 };
+        target.Defend();
+
+        var expectedDamage = monster.Strength - target.Toughness;
+
+        // Act
+        monster.Attack(target);
+
+        // Assert. Health should be more than what we expected damage-wise. But not 100%.
+        Assert.That(target.CurrentHealth, Is.Not.EqualTo(target.TotalHealth));
+        Assert.That(target.CurrentHealth, Is.GreaterThan(target.TotalHealth - expectedDamage));
+    }
+
+    [Test]
+    [TestCase(10)] // equals toughness
+    [TestCase(171)]
+    [TestCase(9999)] // overblock
+    public void Attack_DoesNoDamage_IfDamageIsZeroOrNegative(int strength)
+    {
+        // Arrange
+        var monster = new Monster("Orange Slime", 100, strength, 5);
+        var target = new Character("Ahmed", 100, 100) { Toughness = 9999 };
+
+        // Act
+        monster.Attack(target);
+
+        // Assert
+        Assert.That(target.CurrentHealth, Is.EqualTo(target.TotalHealth));
+    }
+
+    [Test]
+    [TestCase(101)]
+    [TestCase(999)]
+    [TestCase(20717)]
+    public void Attack_SetsTargetHealthToZero_IfTargetDies(int strength)
+    {
+        // Arrange
+        var monster = new Monster("Orange Slime", 1000, strength, 5);
+        var victim = new Character("Ahmed", 100, 100);
+
+        // Act
+        monster.Attack(victim);
+
+        // Assert
+        Assert.That(victim.CurrentHealth, Is.EqualTo(0));
+    }
+}
diff --git a/source/TextBlade.Core/Characters/Monster.cs b/source/TextBlade.Core/Characters/Monster.cs
index 913703b..d5a9b1a 100644
--- a/source/TextBlade.Core/Characters/Monster.cs
+++ b/source/TextBlade.Core/Characters/Monster.cs
@@ -33,6 +33,12 @@ internal int Attack(Character target)
         
         var damage = Math.Max(0, attack - blocked);
         target.CurrentHealth -= damage;
+        
+        if (target.CurrentHealth < 0)
+        {
+            target.CurrentHealth = 0;
+        }
+
         return damage;
     }
 }
diff --git a/source/TextBlade.Core/TextBlade.Core.csproj b/source/TextBlade.Core/TextBlade.Core.csproj
index c108ba4..5fbade0 100644
--- a/source/TextBlade.Core/TextBlade.Core.csproj
+++ b/source/TextBlade.Core/TextBlade.Core.csproj
@@ -17,4 +17,10 @@
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
   </ItemGroup>
 
+  <ItemGroup>
+      <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
+          <_Parameter1>$(AssemblyName).Tests</_Parameter1>
+      </AssemblyAttribute>
+  </ItemGroup>
+
 </Project>