What is external and internal frontier …

Internal :       

• The first set of campaigns along the internal frontier of the Sultanate aimed as consolidating the Hinterland of the Garrison towns.
• During these campaigns forests were cleared in the Ganga _Yamuna doab and hunter gatherers and pastoralists expelled from their habitat.
• Agriculture was encouraged.
• New fotresses and towns were established to protect trade routes and to promote regional trade.    

External :      

• The second expansions occured along the " External " of the sultanate.
• Military expansions into southern India started during the reign of Alauddin Khalji and culminated with Muhammad Tughluq.
• In their campaigns , sultanate armies captured elephants , horses and slaves and carried away precious metals.
• By the end of Muhammad Tuglaq's reign, 150 years after somewhat humble beginings.