Were the Mongols stronger than the Spanish Empire?

It's a tricky question and depends on what you define as strength. Economically no, the Spanish Empire was filthy rich at its peak, at least before the inflation. However I'm going to assume you mean Millitary-wise. This is further complicated depending if you want to consider relative vs absolute strength. read more

The Spanish empire lasted more than 400 years and it only lost the last colonies in the 20th century, they didn’t depend on a military figure, their structures were strong enough. And in terms of influence, today Mongol language and culture is reduced to the former Mongolian territories whereas Spanish is the second most spoken language in the world. read more

Although their empire was short-lived, the Mongols unified much of Eurasia for the first time. They managed to conquer so much territory because their cavalry was more effective than the armies of the sedentary peoples they defeated. After 1400, new gunpowder weapons shifted the military advantage away from nomadic peoples to sedentary societies. read more