Rated 4 for most people, maybe a 5 for history buffs. I was staying on Sullivan's Island for a wedding in Charleston, and virtually stumbled upon this fort, as it was right down the street from where I stayed. Access is easy, parking is plentiful and free, and admission is a mere $3 ($5 per family, $1 for seniors). There are drinks and snacks available in the visitor center.
While Fort Sumter across the harbor gets all the publicity, this fort yields a far better visitor experience. Built to protect Charleston's harbor like Sumter, Moultrie was built in revolutionary times, about 50 years before construction began on Sumter. First built from palmetto logs, it evolved over time. It was abandoned for the stronger Fort Sumter in 1860, but has since been restored to reflect its history over the years.
The visitor center has nice exhibits that contain fine artifacts, and there is a short, informative film about the fort's history. As a bonus, there is an exhibit describing the role of Sullivan's Island in the early slave trade of this country.
Across the street from the visitor center, you can walk through the fort at your liesure (it is self-guided) and view various sections restored to reflect the fort's use over time, from the revolutionary times, through the civil war, and even its use during World Wars I & II as a coastal observation point. You can walk through powder rooms, see what the cannons and shells looked like, and walk on the ramparts where the large guns were mounted. From the ramparts, you can get a good view of the ocean, Fort Sumter and the Charleston harbor. The various sections are well marked, and there are arrows guiding you through the fort.
Outside the fort, you can walk a few hundred yards down a path to the shore, where there are more gun placements added after the civil war, and outstanding views of the harbor. For the time, money and historical content, this attraction is a far better value than Sumter. So, skip the lines and the cost of the ferry, and see this fort. You will be glad you did.