Fort Charles is in Port Royal:
1) Starting from Will Turners Barn (coming out) - turn right and cross the bridge into the grass plains OR If looking at Will Turners Barn (looking at the building from outside) turn left to cross the bridge (past the markets)
2) Reep running as you slowly advance up the hill and left
3) The Fort comes into view, by this time you will have passed a couple of patrol guards - not much of a threat (levels 2 to 7)
4) approaching the fort - there are two guards (Levels 2-7 again) just run by them and up the stairs
5) at the top of the stairs is a dark hallway - a sharp left and you enter the courtyard
You are now in Fort Charles (unlike other forts - many times it does not 'inform' you with a screen message). Proceed to locate crates and barrels around (note - as soon as you search a crate / barrel - the closest guard is 'alerted' to you and will attack... best to bring friends for the upper levels where a few Sargents and Vets levels 12 to 17 hang out).
G/L