Although the assault was a surprise, the troops were nonetheless able to defend the fort. What kind of event was unexpected? Explain.

The assault was unexpected due to it being a surprise.

D; Lee marched over the mountain wall

Explanation:

"...On that pleasant morn of the early fall

Over the mountains winding down,

Horse and foot, into Frederick town.

Forty flags with their silver stars,

Forty flags with their crimson bars,

Flapped in the morning wind: the sun

Of noon looked down, and saw not one.

Up rose old Barbara Frietchie then,

Bowed with her fourscore years and ten;

Bravest of all in Frederick town,

Hope that helps :)