diff --git a/docs-website/docs/server-features/castle-siege.md b/docs-website/docs/server-features/castle-siege.md
index e75787bf5..427bdb371 100644
--- a/docs-website/docs/server-features/castle-siege.md
+++ b/docs-website/docs/server-features/castle-siege.md
@@ -36,3 +36,9 @@ The attacking sides are enemies of each other as well as of the defenders.
 This applies only on the Castle Siege map and only to players with a siege
 side. Killing a player of the own side, or any player outside the battle, is
 still a normal player kill.
+
+## Friendly fire during the battle
+
+During the battle, players of the same siege side can't damage each other,
+including with area skills and pets. This applies only on the Castle Siege map
+and only to players with a siege side.
diff --git a/src/GameLogic/CastleSiege/CastleSiegeContext.cs b/src/GameLogic/CastleSiege/CastleSiegeContext.cs
index 36947be0b..28ad8d89f 100644
--- a/src/GameLogic/CastleSiege/CastleSiegeContext.cs
+++ b/src/GameLogic/CastleSiege/CastleSiegeContext.cs
@@ -263,6 +263,24 @@ public bool AreOnOpposingSides(Player player, Player otherPlayer)
                && side != otherSide;
     }
 
+    /// <summary>
+    /// Determines whether two players fight on the same side of the running battle.
+    /// </summary>
+    /// <param name="player">The one player.</param>
+    /// <param name="otherPlayer">The other player.</param>
+    /// <returns><see langword="true"/> if both are on the Castle Siege map during the battle, and on the same side.</returns>
+    public bool AreOnSameSide(Player player, Player otherPlayer)
+    {
+        if (this.CurrentState != CastleSiegeState.Start)
+        {
+            return false;
+        }
+
+        var side = this.GetPlayerJoinSide(player);
+        return side != CastleSiegeJoinSide.None
+               && side == this.GetPlayerJoinSide(otherPlayer);
+    }
+
     /// <summary>
     /// Assigns the current Castle Siege side to all online players on the Castle Siege map.
     /// </summary>
diff --git a/src/GameLogic/CastleSiege/CastleSiegePlugIn.cs b/src/GameLogic/CastleSiege/CastleSiegePlugIn.cs
index 204b950d4..5bf70dcf3 100644
--- a/src/GameLogic/CastleSiege/CastleSiegePlugIn.cs
+++ b/src/GameLogic/CastleSiege/CastleSiegePlugIn.cs
@@ -4,6 +4,7 @@
 
 namespace MUnique.OpenMU.GameLogic.CastleSiege;
 
+using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Threading;
@@ -17,11 +18,12 @@ namespace MUnique.OpenMU.GameLogic.CastleSiege;
 /// </summary>
 /// <remarks>
 /// During the battle, the attacks and kills between players of opposing sides are exempted from the player killer penalty.
+/// During the battle, players of the same side can't damage each other.
 /// </remarks>
 [PlugIn]
 [Display(Name = nameof(PlugInResources.CastleSiegePlugIn_Name), Description = nameof(PlugInResources.CastleSiegePlugIn_Description), ResourceType = typeof(PlugInResources))]
 [Guid("B7F62FA9-59E6-49E9-B499-0358A14957CF")]
-public class CastleSiegePlugIn : IPeriodicTaskPlugIn, IObjectAddedToMapPlugIn, IObjectRemovedFromMapPlugIn, IPlayerStateChangedPlugIn, IPlayerSpawnGateSelectionPlugIn, IPlayerKillPenaltyExemptionPlugIn
+public class CastleSiegePlugIn : IPeriodicTaskPlugIn, IObjectAddedToMapPlugIn, IObjectRemovedFromMapPlugIn, IPlayerStateChangedPlugIn, IPlayerSpawnGateSelectionPlugIn, IPlayerKillPenaltyExemptionPlugIn, IPlayerAttackRestrictionPlugIn
 {
     private static readonly TimeSpan EconomySaveInterval = TimeSpan.FromSeconds(30);
     private static readonly TimeSpan NpcSaveInterval = TimeSpan.FromMinutes(2);
@@ -138,6 +140,16 @@ public ValueTask SelectSpawnGateAsync(Player player, SpawnGateSelectionArgs args
         return ValueTask.CompletedTask;
     }
 
+    /// <inheritdoc />
+    public void CheckAttack(Player attacker, Player defender, CancelEventArgs eventArgs)
+    {
+        if (this.GetContext(attacker.GameContext) is { } context
+            && context.AreOnSameSide(attacker, defender))
+        {
+            eventArgs.Cancel = true;
+        }
+    }
+
     /// <inheritdoc />
     public void CheckExemption(Player attacker, Player defender, PlayerKillPenaltyExemptionArgs eventArgs)
     {
diff --git a/src/GameLogic/Player.cs b/src/GameLogic/Player.cs
index b620dde83..2dd61d592 100644
--- a/src/GameLogic/Player.cs
+++ b/src/GameLogic/Player.cs
@@ -5,6 +5,7 @@
 namespace MUnique.OpenMU.GameLogic;
 
 using System;
+using System.ComponentModel;
 using System.Globalization;
 using System.Threading;
 using MUnique.OpenMU.AttributeSystem;
@@ -785,7 +786,8 @@ public async ValueTask KillInstantlyAsync()
             throw new InvalidOperationException("AttributeSystem not set.");
         }
 
-        if (this.IsAttackBlockedBySafezone(attacker))
+        if (this.IsAttackBlockedBySafezone(attacker)
+            || this.IsAttackPreventedByPlugIns(attacker))
         {
             return null;
         }
@@ -2147,6 +2149,25 @@ private async void OnTransformationSkinChanged(object? sender, EventArgs args)
         }
     }
 
+    /// <summary>
+    /// Determines whether a plugin prevents the attacking player, or the owner of the attacking pet, from damaging this player.
+    /// </summary>
+    /// <param name="attacker">The attacker.</param>
+    /// <returns><c>true</c>, if the attack is prevented; otherwise, <c>false</c>.</returns>
+    private bool IsAttackPreventedByPlugIns(IAttacker attacker)
+    {
+        if ((attacker as Player ?? (attacker as IPlayerSurrogate)?.Owner) is not { } attackerPlayer
+            || attackerPlayer == this
+            || this.GameContext.PlugInManager.GetPlugInPoint<IPlayerAttackRestrictionPlugIn>() is not { } plugInPoint)
+        {
+            return false;
+        }
+
+        var eventArgs = new CancelEventArgs();
+        plugInPoint.CheckAttack(attackerPlayer, this, eventArgs);
+        return eventArgs.Cancel;
+    }
+
     /// <summary>
     /// Sets the reclaimable attributes before a character enters the game.
     /// Current shield and mana are set to their maximum values.
diff --git a/src/GameLogic/PlugIns/IPlayerAttackRestrictionPlugIn.cs b/src/GameLogic/PlugIns/IPlayerAttackRestrictionPlugIn.cs
new file mode 100644
index 000000000..d1ae91092
--- /dev/null
+++ b/src/GameLogic/PlugIns/IPlayerAttackRestrictionPlugIn.cs
@@ -0,0 +1,26 @@
+// <copyright file="IPlayerAttackRestrictionPlugIn.cs" company="MUnique">
+// Licensed under the MIT License. See LICENSE file in the project root for full license information.
+// </copyright>
+
+namespace MUnique.OpenMU.GameLogic.PlugIns;
+
+using System.ComponentModel;
+using System.Runtime.InteropServices;
+using MUnique.OpenMU.PlugIns;
+
+/// <summary>
+/// Plugins which can prevent a player from damaging another player,
+/// e.g. between the players of the same side of a Castle Siege battle.
+/// </summary>
+[Guid("9130ECFA-9916-4074-92DC-5AEA17BC4F1A")]
+[PlugInPoint("Player attack restriction", "Plugins which can prevent a player from damaging another player.")]
+public interface IPlayerAttackRestrictionPlugIn
+{
+    /// <summary>
+    /// Checks if the attacker may damage the defender.
+    /// </summary>
+    /// <param name="attacker">The attacking player, or the owner of the attacking pet.</param>
+    /// <param name="defender">The attacked player.</param>
+    /// <param name="eventArgs">The event args. A plugin sets <see cref="CancelEventArgs.Cancel"/> to prevent the attack.</param>
+    void CheckAttack(Player attacker, Player defender, CancelEventArgs eventArgs);
+}
diff --git a/tests/MUnique.OpenMU.Tests/CastleSiegeFriendlyFireTests.cs b/tests/MUnique.OpenMU.Tests/CastleSiegeFriendlyFireTests.cs
new file mode 100644
index 000000000..d1b412184
--- /dev/null
+++ b/tests/MUnique.OpenMU.Tests/CastleSiegeFriendlyFireTests.cs
@@ -0,0 +1,145 @@
+// <copyright file="CastleSiegeFriendlyFireTests.cs" company="MUnique">
+// Licensed under the MIT License. See LICENSE file in the project root for full license information.
+// </copyright>
+
+namespace MUnique.OpenMU.Tests;
+
+using MUnique.OpenMU.DataModel.Configuration;
+using MUnique.OpenMU.GameLogic;
+using MUnique.OpenMU.GameLogic.Attributes;
+using MUnique.OpenMU.GameLogic.CastleSiege;
+using MUnique.OpenMU.GameLogic.PlugIns;
+using MUnique.OpenMU.Interfaces;
+using MUnique.OpenMU.Pathfinding;
+using MUnique.OpenMU.PlugIns;
+using BasicModel = MUnique.OpenMU.Persistence.BasicModel;
+
+/// <summary>
+/// Tests that players of the same side of a Castle Siege battle can't damage each other.
+/// </summary>
+[TestFixture]
+public class CastleSiegeFriendlyFireTests
+{
+    private static readonly Guid CastleSiegePlugInId = new("B7F62FA9-59E6-49E9-B499-0358A14957CF");
+
+    /// <summary>
+    /// Verifies that only players of the same side on the siege map during the battle are on the same side.
+    /// </summary>
+    [Test]
+    public async ValueTask OnlyTheSameSideDuringTheBattleIsFriendlyAsync()
+    {
+        var fixture = await SiegeFixture.CreateAsync().ConfigureAwait(false);
+        var attacker = await fixture.CreatePlayerAsync(1, CastleSiegeJoinSide.Attack1).ConfigureAwait(false);
+        var ally = await fixture.CreatePlayerAsync(2, CastleSiegeJoinSide.Attack1).ConfigureAwait(false);
+        var otherAttacker = await fixture.CreatePlayerAsync(3, CastleSiegeJoinSide.Attack2).ConfigureAwait(false);
+        var spectator = await fixture.CreatePlayerAsync(4, CastleSiegeJoinSide.None).ConfigureAwait(false);
+        var otherSpectator = await fixture.CreatePlayerAsync(5, CastleSiegeJoinSide.None).ConfigureAwait(false);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(fixture.Context.AreOnSameSide(attacker, ally), Is.True);
+            Assert.That(fixture.Context.AreOnSameSide(attacker, otherAttacker), Is.False, "Different attacking sides.");
+            Assert.That(fixture.Context.AreOnSameSide(attacker, spectator), Is.False, "The spectator has no side.");
+            Assert.That(fixture.Context.AreOnSameSide(spectator, otherSpectator), Is.False, "Players without side aren't a side.");
+        });
+
+        ally.SetCurrentMapSilently(fixture.OtherMap);
+        Assert.That(fixture.Context.AreOnSameSide(attacker, ally), Is.False, "The ally left the siege map.");
+
+        ally.SetCurrentMapSilently(fixture.Map);
+        fixture.Context.CurrentState = CastleSiegeState.End;
+        Assert.That(fixture.Context.AreOnSameSide(attacker, ally), Is.False, "Outside of the battle.");
+    }
+
+    /// <summary>
+    /// Verifies that an attack on a player of the same side doesn't hit.
+    /// </summary>
+    [Test]
+    public async ValueTask AttackOnTheSameSideIsPreventedAsync()
+    {
+        var fixture = await SiegeFixture.CreateAsync().ConfigureAwait(false);
+        var attacker = await fixture.CreatePlayerAsync(1, CastleSiegeJoinSide.Attack1).ConfigureAwait(false);
+        var ally = await fixture.CreatePlayerAsync(2, CastleSiegeJoinSide.Attack1).ConfigureAwait(false);
+        var healthBefore = ally.Attributes![Stats.CurrentHealth];
+
+        var hitInfo = await ally.AttackByAsync(attacker, null, false).ConfigureAwait(false);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(hitInfo, Is.Null);
+            Assert.That(ally.Attributes[Stats.CurrentHealth], Is.EqualTo(healthBefore));
+        });
+    }
+
+    /// <summary>
+    /// Verifies that an attack on a player of an opposing side still hits.
+    /// </summary>
+    [Test]
+    public async ValueTask AttackOnAnOpposingSideIsNotPreventedAsync()
+    {
+        var fixture = await SiegeFixture.CreateAsync().ConfigureAwait(false);
+        var attacker = await fixture.CreatePlayerAsync(1, CastleSiegeJoinSide.Attack1).ConfigureAwait(false);
+        var defender = await fixture.CreatePlayerAsync(2, CastleSiegeJoinSide.Defense).ConfigureAwait(false);
+
+        var hitInfo = await defender.AttackByAsync(attacker, null, false).ConfigureAwait(false);
+
+        Assert.That(hitInfo, Is.Not.Null);
+    }
+
+    private sealed class SiegeFixture
+    {
+        public SiegeFixture(IGameContext gameContext, CastleSiegeContext context, GameMap map, GameMap otherMap)
+        {
+            this.GameContext = gameContext;
+            this.Context = context;
+            this.Map = map;
+            this.OtherMap = otherMap;
+        }
+
+        public IGameContext GameContext { get; }
+
+        public CastleSiegeContext Context { get; }
+
+        public GameMap Map { get; }
+
+        public GameMap OtherMap { get; }
+
+        public static async ValueTask<SiegeFixture> CreateAsync()
+        {
+            var gameContext = GameContextTestHelper.CreateGameContext(
+            [
+                new PlugInConfiguration { TypeId = CastleSiegePlugInId, IsActive = true },
+            ]);
+            var siegeMap = new BasicModel.GameMapDefinition { Number = 30, TerrainData = new byte[ushort.MaxValue + 3] };
+            gameContext.Configuration.Maps.Add(siegeMap);
+            var configuration = new BasicModel.CastleSiegeConfiguration
+            {
+                Enabled = true,
+                CastleSiegeMapDefinition = siegeMap,
+            };
+            configuration.StateSchedule.Add(new BasicModel.CastleSiegeStateScheduleEntry { State = CastleSiegeState.Ready, DayOfWeek = DayOfWeek.Monday });
+            configuration.StateSchedule.Add(new BasicModel.CastleSiegeStateScheduleEntry { State = CastleSiegeState.Start, DayOfWeek = DayOfWeek.Tuesday });
+            gameContext.Configuration.CastleSiegeConfiguration = configuration;
+
+            var plugIn = gameContext.PlugInManager.GetActivePlugInsOf<IPeriodicTaskPlugIn>().OfType<CastleSiegePlugIn>().Single();
+            await plugIn.ExecuteTaskAsync(gameContext).ConfigureAwait(false);
+            var context = plugIn.GetContext(gameContext) ?? throw new InvalidOperationException("The context wasn't created.");
+            context.CurrentState = CastleSiegeState.Start;
+
+            var map = new GameMap(siegeMap, TimeSpan.FromMinutes(1), 8);
+            var otherMap = new GameMap(gameContext.Configuration.Maps.First(), TimeSpan.FromMinutes(1), 8);
+            return new SiegeFixture(gameContext, context, map, otherMap);
+        }
+
+        public async ValueTask<Player> CreatePlayerAsync(uint guildId, CastleSiegeJoinSide side)
+        {
+            var player = await PlayerTestHelper.CreatePlayerAsync(this.GameContext).ConfigureAwait(false);
+            player.SelectedCharacter!.Id = Guid.NewGuid();
+            player.GuildStatus = new GuildMemberStatus(guildId, GuildPosition.NormalMember);
+            player.Position = new Point((byte)guildId, 20);
+            player.SetCurrentMapSilently(this.Map);
+            this.Context.PlayerJoinSides[player.SelectedCharacter.Id] = side;
+            return player;
+        }
+    }
+}