In 2:14-16 Isaiah lists things that God will destroy. Possible meanings are 1) These refer to prideful people that God will humble or 2) God will actually destroy all these things in the list.
People often built their cities on mountains or hills to help protect them from enemies. Also, people believed their false gods lived on mountains and large hills so they would go there to worship.
## high tower...impregnable wall ##
These refer to things built around a city to defend the people living in it from their enemies.
## ships of Tarshish...beautiful sailing vessels ##
These refer to large boats that people used to travele far on the sea and bring back riches to the cities.