Charlemagne's Knights (2005)

From the introduction:

In the summer of 809 AD a new and terrible force entered the world. Deep in the Pyrenees Mountains between France and Spain, a mystical portal opened up, linking the Earth to one of the many worlds of the Gromler Empire. Through this portal marched the greatest army the world has ever seen. Many tens of thousands of Gromler soldiers poured through the portal, supported by their massive armored tanks, each nearly as big a fortress. Supporting this mighty army were also hundreds of Gromler Wizards, the best of which used mystical powers like the greatest of sorcerors. The Gromler army possessed many military wonders, such as steam driven tanks, sophisticated guns, explosive grenades and worse. Charlemagne’s local barons and the military garrisons they could field were no match for this great force as the Gromler army quickly spread north and south out of the mountains.