diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e2e0dd9
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,100 @@
+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
+
+# C extensions
+*.so
+
+# Distribution / packaging
+.Python
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+wheels/
+share/python-wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+MANIFEST
+
+# PyInstaller
+*.manifest
+*.spec
+
+# Installer logs
+pip-log.txt
+pip-delete-this-directory.txt
+
+# Unit test / coverage reports
+htmlcov/
+.tox/
+.nox/
+.coverage
+.coverage.*
+.cache
+nosetests.xml
+coverage.xml
+*.cover
+*.py,cover
+.hypothesis/
+.pytest_cache/
+cover/
+
+# Translations
+*.mo
+*.pot
+
+# Documentation builds (Doxygen writes to build/doxygen, see Doxyfile)
+docs/_build/
+doxygen_warnings.log
+
+# Jupyter Notebook
+.ipynb_checkpoints
+
+# IPython
+profile_default/
+ipython_config.py
+
+# pyenv
+.python-version
+
+# Environments
+.env
+.envrc
+.venv
+env/
+venv/
+ENV/
+env.bak/
+venv.bak/
+
+# Type checkers and linters
+.mypy_cache/
+.dmypy.json
+dmypy.json
+.pyre/
+.pytype/
+.ruff_cache/
+
+# IDEs and editors
+.idea/
+.vscode/
+*.swp
+*.swo
+.spyderproject
+.spyproject
+.ropeproject
+
+# Operating system files
+.DS_Store
+Thumbs.db
+desktop.ini
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..da07083
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,48 @@
+[build-system]
+requires = ["setuptools>=77"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "turtle-challenges"
+version = "0.1.0"
+description = "Turtle graphics exercises from Udemy's 100 Days of Code Python bootcamp (day 18)."
+readme = "README.md"
+requires-python = ">=3.13"
+license = "AGPL-3.0-only"
+license-files = ["LICENSE"]
+# Only the standard library is used at run time (turtle needs Tk, see README).
+dependencies = []
+classifiers = [
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.13",
+    "Topic :: Education",
+]
+
+[project.optional-dependencies]
+dev = [
+    "mypy>=1.13",
+    "pytest>=8.3",
+    "ruff>=0.8",
+]
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+pythonpath = ["src", "."]
+testpaths = ["tests"]
+
+[tool.ruff]
+line-length = 88
+target-version = "py313"
+src = ["src", "tests"]
+# Only the project's own code: the framework submodule and agent skills are not ours.
+include = ["src/**/*.py", "tests/**/*.py"]
+
+[tool.ruff.lint]
+select = ["ANN", "B", "E", "F", "I", "N", "SIM", "UP", "W"]
+
+[tool.mypy]
+strict = true
+files = ["src", "tests"]
+mypy_path = "src"
diff --git a/src/turtle_challenges/__init__.py b/src/turtle_challenges/__init__.py
new file mode 100644
index 0000000..fda5d58
--- /dev/null
+++ b/src/turtle_challenges/__init__.py
@@ -0,0 +1,7 @@
+"""!
+@file __init__.py
+@brief Solutions to the turtle graphics challenges of day 18 of Udemy's
+100 Days of Code.
+
+The challenges are added one milestone at a time; see the README.
+"""
diff --git a/src/turtle_challenges/constants.py b/src/turtle_challenges/constants.py
new file mode 100644
index 0000000..cb2fb0a
--- /dev/null
+++ b/src/turtle_challenges/constants.py
@@ -0,0 +1,15 @@
+"""!
+@file constants.py
+@brief Every constant of the project, in one place.
+
+Numbers and names used by the challenges live here and nowhere else, so a
+challenge can be tuned (for example the number of dashes) without touching its
+drawing code.
+"""
+
+## Title of the turtle window.
+WINDOW_TITLE = "Turtle Challenges"
+
+## Largest value of one color channel; also the turtle color mode, so that
+## colors can be given as RGB tuples from 0 to 255.
+COLOR_MODE = 255
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/test_package.py b/tests/test_package.py
new file mode 100644
index 0000000..0470738
--- /dev/null
+++ b/tests/test_package.py
@@ -0,0 +1,14 @@
+import turtle_challenges
+from turtle_challenges import constants
+
+
+def test_package_can_be_imported_and_is_documented() -> None:
+    assert turtle_challenges.__doc__
+
+
+def test_color_mode_is_the_rgb_range_of_the_course() -> None:
+    assert constants.COLOR_MODE == 255
+
+
+def test_window_title_is_not_empty() -> None:
+    assert constants.WINDOW_TITLE