Northgard is described as 'Take control of a Viking clan in this real-time strategy game based on Norse mythology'. There are four games similar to Northgard for a variety of platforms, including Windows, Steam, Linux, Android and Android Tablet. The best alternative is 0 A.D., which is both free and Open Source. Other Northgard like games are Age of Mythology (Paid), Rusted Warfare (Paid) and Land and Castles (Freemium).
0 A.D. is a free, open-source, cross-platform real-time strategy (RTS) game of ancient warfare. In short, it is a historically-based war/economy game that allows players to relive or rewrite the history of Western civilizations, focusing on the years between 500 B.C. and 500 A.D.
Age of Mythology is based on building towns, gathering resources, creating armies, and ultimately destroying enemy units and buildings. In this way, players are able to defeat and conquer rival towns and civilizations.