Ft Sumter is more than just the site of the beginning of the Civil War; it and others have guarded Charleston Harbor since the 1700s. It completely occupies a tiny island right at the harbor's narrow mouth. Once there, you can see how its location was so vital to the security of one of America's first cities. In fact, it was an active coastal defense through World War II.
As for its most well known footnote in history, when South Carolina made the decision to secede from the Union, the Federal Army still occupied the city's defenses, and while the army's loyalty was to the north, the town had become their home. They never received instructions to fire on the town, and it is doubtful they would have carried out orders if they had. The city soon decided to force the issue and fired on the fortress until it surrendered, still unwilling to fight back. It became a symbol to both sides for the next 5 years of conflict, and still embodies gallantry or rebellion for many, depending whose side you are on. Union forces eventually recaptured it, but not without tremendous effort. However, the fort is so well built and so far from its attackers that very few casualties were suffered.
The cannon ports have all been sealed for safety, and there's a few places you can't walk, but more than enough to explore. Here you can see the metal arches that were the tracks the cannon were angled on. If you look closely at them, you can see the degrees marked.
If you want to touch a piece of history, you can find several places where projectiles are embedded in the walls from the many bombardments the fort endured.
When you are tired of the weather (which is tempered or exacerbated by a near constant ocean wind), you can meander through the air conditioned museum, It's small, but packed full of history. Unfortunately, my little ones didn't appreciate it much, which made it harder for me to as well.