Libre Tower is a project inspired by the likes of Freedoom to make a free, open-source Pizza Tower-like game. Not just free as in no cost, but as stated by Freedoom's own readme, free to copy, modify, and redistribute without needed permission, as long as it is shared alike/with the same license.
The raw assets for the project can be found at Applemunch/LibreTower-Assets.
If you'd like to get started for contributing to Libre Tower, feel free to send pull requests.