diff --git a/.github/workflows/testing.yml b/.github/workflows/testing.yml
index dfcee3fb7..5d1a36319 100644
--- a/.github/workflows/testing.yml
+++ b/.github/workflows/testing.yml
@@ -320,3 +320,38 @@ jobs:
         shell: bash
         run: |
           test/run-tests.sh
+
+  cygwin:
+    strategy:
+      matrix:
+        bits: [32, 64]
+        include:
+          - bits: 32
+            arch: x86
+          - bits: 64
+            arch: x86_x64
+      fail-fast: false
+    runs-on: windows-2019
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+      - name: Setup Cygwin
+        uses: cygwin/cygwin-install-action@v4
+        with:
+          platform: ${{ matrix.arch }}
+          packages: >-
+            bison
+            gcc-core
+            git
+            libpng-devel
+            make
+            pkg-config
+      - name: Build & install using Make
+        run: |
+          PATH="/usr/bin:$(cygpath "${SYSTEMROOT}")/System32"
+          make develop -kj Q=
+          make install -j Q=
+      - name: Run tests
+        run: |
+          PATH="/usr/bin:$(cygpath "${SYSTEMROOT}")/System32"
+          test/run-tests.sh --only-internal
diff --git a/test/run-tests.sh b/test/run-tests.sh
index d4a226893..e82094287 100755
--- a/test/run-tests.sh
+++ b/test/run-tests.sh
@@ -6,13 +6,15 @@ cd "$(dirname "$0")"
 usage() {
 	echo "Runs regression tests on RGBDS."
 	echo "Options:"
-	echo "    -h, --help      show this help message"
-	echo "    --only-free     skip tests that build nonfree codebases"
+	echo "    -h, --help          show this help message"
+	echo "    --only-free         skip tests that build nonfree codebases"
+	echo "    --only-internal     skip tests that build external codebases"
 }
 
 # Parse options in pure Bash because macOS `getopt` is stuck
 # in what util-linux `getopt` calls `GETOPT_COMPATIBLE` mode
 nonfree=true
+external=true
 FETCH_TEST_DEPS="fetch-test-deps.sh"
 while [[ $# -gt 0 ]]; do
 	case "$1" in
@@ -24,6 +26,9 @@ while [[ $# -gt 0 ]]; do
 			nonfree=false
 			FETCH_TEST_DEPS="fetch-test-deps.sh --only-free"
 			;;
+		--only-internal)
+			external=false
+			;;
 		--)
 			break
 			;;
@@ -49,6 +54,10 @@ for dir in asm link fix gfx; do
 	popd
 done
 
+if ! "$external"; then
+	exit
+fi
+
 # Test some significant external projects that use RGBDS
 # When adding new ones, don't forget to add them to the .gitignore!
 # When updating subprojects, change the commit being checked out, and set the `shallow-since`