FlatRedBall is an open source C# based game engine with development dating back to 2005. It was originally built to run on-top of Managed Direct X, then was ported to XNA and when XNA was depreciated, it was again ported to run on top of the MonoGame framework.
FlatRedBall provides a layer of APIs and tooling on top of MonoGame designed to simplify the process of creating 2D games. You can currently create games for Windows (and UWP), Android and iOS, with Mac and Linux targets currently a work in progress. The heart of the tooling is Glue, which “glues” together the various other tools, including plugins for tasks such as UI development as well as support for the Tiled 2D map editor.
FlatRedBall is open source with the source code available on GitHub under the flexible and permissive MIT open source license. You can check out FlatRedBall in action in the video below (or here on Odysee). If you are interested in learning more or encounter a problem, they have an active Discord server available here.