Here's an open-source clone of Civilization II. Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization.

Here's a neat open-source (slow-build) RTS that's similar to Settlers II. It focuses on developing your settlement, yet also has combat. Widelands has a single-player campaign as well as multiplayer.

Futuristic open-source RTS. Available through Steam and itch.io.

Another free, open-source RTS based on the Spring engine. Available through Steam and itch.io.