diff --git a/niar/__init__.py b/niar/__init__.py
index f1888ec..aa239e1 100644
--- a/niar/__init__.py
+++ b/niar/__init__.py
@@ -17,7 +17,7 @@ def cli(np: Project):
             "build", help="build the design, and optionally program it"
         ),
     )
-    if hasattr(np, "cxxrtl_targets"):
+    if np.cxxrtl_targets:
         cxxrtl.add_arguments(
             np, subparsers.add_parser("cxxrtl", help="run the C++ simulator tests")
         )
diff --git a/niar/build.py b/niar/build.py
index 3cdfb06..1918459 100644
--- a/niar/build.py
+++ b/niar/build.py
@@ -62,6 +62,9 @@ def main(np: Project, args):
             "yosys_opts": "-g",
         }
         prepare_kwargs.update(getattr(platform, "prepare_kwargs", {}))
+        for p in np.externals:
+            with open(np.path(p), 'rb') as f:
+                platform.add_file(p, f)
         plan = platform.prepare(design, np.name, **prepare_kwargs)
 
     il_fn = f"{np.name}.il"
diff --git a/niar/project.py b/niar/project.py
index 50920f0..ff84eea 100644
--- a/niar/project.py
+++ b/niar/project.py
@@ -82,7 +82,8 @@ class Project:
     name: str
     top: type[Elaboratable]
     targets: list[type[Platform]]
-    cxxrtl_targets: Optional[list[type[CxxrtlPlatform]]]
+    cxxrtl_targets: list[type[CxxrtlPlatform]] = []
+    externals: list[str] = []
 
     origin: Path
 
@@ -149,7 +150,7 @@ class Project:
         raise KeyError(f"unknown target {name!r}")
 
     def cxxrtl_target_by_name(self, name: str) -> CxxrtlPlatform:
-        for t in self.cxxrtl_targets or []:
+        for t in self.cxxrtl_targets:
             if t.__name__ == name:
                 return t()
         raise KeyError(f"unknown CXXRTL target {name!r}")