What is a fortification?
1.wall
2.moat
3.tower
4.bridge

wall

Explanation:

a strong wall or other reinforcement built to protect a place against attack. or   the action of building or process of being built.

The cause of the U.S. Participation was pirates from the Barbary States seizing American merchant ships and holding the crews for ransom, demanding the U.S. pay tribute to the Barbary rulers. United States President Thomas Jefferson refused to pay this tribute. Sweden had been at war with the Tripolitans since 1800