diff --git a/Kernel/Arch/x86/PageDirectory.h b/Kernel/Arch/x86/PageDirectory.h
index 12278e422c..f0db3a22e6 100644
--- a/Kernel/Arch/x86/PageDirectory.h
+++ b/Kernel/Arch/x86/PageDirectory.h
@@ -11,6 +11,9 @@
 #include <Kernel/Forward.h>
 #include <Kernel/PhysicalAddress.h>
 
+#include <AK/Platform.h>
+VALIDATE_IS_X86()
+
 namespace Kernel {
 
 class PageDirectoryEntry {
diff --git a/Kernel/BootInfo.h b/Kernel/BootInfo.h
index 6ecc813788..48330cce7d 100644
--- a/Kernel/BootInfo.h
+++ b/Kernel/BootInfo.h
@@ -6,11 +6,14 @@
 
 #pragma once
 
-#include <Kernel/Arch/x86/PageDirectory.h>
 #include <Kernel/Multiboot.h>
 #include <Kernel/PhysicalAddress.h>
 #include <Kernel/VirtualAddress.h>
 
+namespace Kernel {
+class PageTableEntry;
+}
+
 extern "C" PhysicalAddress start_of_prekernel_image;
 extern "C" PhysicalAddress end_of_prekernel_image;
 extern "C" size_t physical_to_virtual_offset;
diff --git a/Kernel/Memory/MemoryManager.h b/Kernel/Memory/MemoryManager.h
index edd6dd1640..fd75a08053 100644
--- a/Kernel/Memory/MemoryManager.h
+++ b/Kernel/Memory/MemoryManager.h
@@ -21,6 +21,10 @@
 #include <Kernel/Memory/Region.h>
 #include <Kernel/Memory/VMObject.h>
 
+namespace Kernel {
+class PageDirectoryEntry;
+}
+
 namespace Kernel::Memory {
 
 constexpr bool page_round_up_would_wrap(FlatPtr x)