How was the mongol period was both constructive and destructive ?
The Mongol period was destructive as the Mongols destroyed many cities and towns in their paths, particularly those who refused to submit to Mongol rule. Of particular note, the Mongols destroyed the the Song dynasty in China and the Abbasid dynasty of the Middle East. Because they did not understand farming as a way of life, they destroyed canals and irrigation systems in areas which they conquered.
At the same times, the Mongols were constructive in that they ensured the safety of the silk roads and thus allowed trade to flourish along the routes. The Mongols greatly reduced the threat of bandits on the roads, and trade increased substantially. This had the effect of increasing the contact between East and West.
Mongols were especially influential in the diffusion of the use of gunpowder; although one might argue whether this was destructive or constructive. The armies of Ghengis Khan learned the use of gunpowder from the Chinese and used it to attack fortified cities. the introduction of gunpowder and fire arms marked the end of walled cities as a means of defense.