diff --git a/.github/sc4pac-yaml-schema.py b/.github/sc4pac-yaml-schema.py
index f043ceef..11a418d2 100644
--- a/.github/sc4pac-yaml-schema.py
+++ b/.github/sc4pac-yaml-schema.py
@@ -111,6 +111,34 @@
 }
 
 
+class DependencyChecker:
+    def __init__(self):
+        self.known_packages = set()
+        self.known_assets = set()
+        self.referenced_packages = set()
+        self.referenced_assets = set()
+
+    def aggregate_identifiers(self, doc):
+        if 'assetId' in doc:
+            self.known_assets.add(doc['assetId'])
+        if 'group' in doc and 'name' in doc:
+            self.known_packages.add(doc['group'] + ":" + doc['name'])
+            self.referenced_packages.update(doc.get('dependencies', []))
+            self.referenced_assets.update(
+                    a['assetId'] for a in doc.get('assets', [])
+                    if 'assetId' in a)
+            for v in doc.get('variants', []):
+                self.referenced_packages.update(v.get('dependencies', []))
+                self.referenced_assets.update(
+                        a['assetId'] for a in v.get('assets', [])
+                        if 'assetId' in a)
+
+    def unknowns(self):
+        packages = sorted(self.referenced_packages.difference(self.known_packages))
+        assets = sorted(self.referenced_assets.difference(self.known_assets))
+        return {'packages': packages, 'assets': assets}
+
+
 def main() -> int:
     args = sys.argv[1:]
     if not args:
@@ -121,6 +149,7 @@ def main() -> int:
     from jsonschema import exceptions
     validator = Draft202012Validator(schema)
     validator.check_schema(schema)
+    dependencyChecker = DependencyChecker()
     validated = 0
     errors = 0
     for d in args:
@@ -133,11 +162,33 @@ def main() -> int:
                     validated += 1
                     text = f.read()
                     for doc in yaml.safe_load_all(text):
+                        dependencyChecker.aggregate_identifiers(doc)
                         err = exceptions.best_match(validator.iter_errors(doc))
-                        if err is not None:
+                        msgs = [] if err is None else [err.message]
+
+                        # check URLs
+                        urls = [u for u in [doc.get('url'), doc.get('info', {}).get('website')]
+                                if u is not None]
+                        for u in urls:
+                            if '/sc4evermore.com/' in u:
+                                msgs.append(f"Domain of URL {u} should be www.sc4evermore.com")
+
+                        if msgs:
                             errors += 1
                             print(f"===> {p}")
-                            print(err.message)
+                            for msg in msgs:
+                                print(msg)
+
+    if not errors:
+        # check that all dependencies exist
+        # (this check only makes sense for the self-contained main channel)
+        for label, unknown in dependencyChecker.unknowns().items():
+            if unknown:
+                errors += len(unknown)
+                print(f"===> The following {label} are referenced, but not defined:")
+                for identifier in unknown:
+                    print(identifier)
+
     if errors > 0:
         print(f"Finished with {errors} errors.")
         return 1