Fort Donelson originally covered 15 acres. Confederate soldiers and enslaved African Americans built the fort over seven months, raising log-and-earth walls 10 feet high. Though less permanent than brick or stone, earthen walls gave better protection against artillery fire. Confederates built the fort to protect upper and lower batteries along the Cumberland River from land attack, but the fort never came under direct land attack. Except for ironclad gunboat attacks on the river batteries, all fighting took place in the outer defense lines. Brigadier General Daniel S. Donelson, for whom the fort was named, took part in the original 1861 survey that selected the sites for both Forts Henry and Donelson.