HHHMMM arkadaşın mütevazi yardımıyla 3 alt kategoriye kadar entegre ettik.

<nav class="header__navigation collapse navbar-toggleable-md" id="structurepress-main-navigation" aria-label="Main Menu">

	<a class="home-icon" href="index.html">
		<i class="fa fa-home"></i>
	</a>

	<ul class="main-navigation js-main-nav" role="menubar">
		@{
			List<Category> AllCategory = Model.FirstOrDefault().Category;

			List<Category> AnaMenu = AllCategory.Where(x => x.ParentId == 11).OrderBy(x => x.CategoryOrder).ToList();

			foreach (var item in AnaMenu)
			{
				List<Category> AltMenu = AllCategory.Where(x => x.ParentId == item.CategoryId).ToList();

				<li class=@(AltMenu.Count > 0 ? "menu-item-has-children" : "")>
					<a href="@item.CategoryId">@item.CategoryName</a>

					@if (AltMenu.Count > 0)
					{
						<ul class="sub-menu">
						@{ 
							List<Category> AltMenu2 = AllCategory.Where(x => x.ParentId == item.CategoryId).ToList();
							foreach (var item2 in AltMenu2)
							{
								<li>
									<a href="@item2.CategoryId">@item2.CategoryName</a>
									@if (AltMenu2.Count > 0)
									{
									<ul class="sub-menu">
										@{
											List<Category> AltMenu3 = AllCategory.Where(x => x.ParentId == item2.CategoryId).ToList();
											foreach (var item3 in AltMenu3)
											{
												<li>
													<a href="@item3.CategoryId">@item3.CategoryName</a>
												</li>
											}
										 }
									 </ul>
									}
								</li>
							}
						}
						</ul>
					}
				</li>
			}
		}

	</ul>

</nav>