Alright, so, there are three ways to go about getting that door open.
Way #1 is the most common, it involves sending a unit to the fort, and that opens the gate.
Way #2 is the same in concept. If you slaughtered Gordon as an enemy unit, it opens.
Way #3 is silly. Send every unit but Marth to fight the knights. Once they all die, it opens.
Also, about the two doors.
Where Gordin is there are two doors. One to where you start, and one to where the seize point is. On your side of the prison, there's an archer (?) that has a door key, that lets you recruit Gordin. After doing so, the knights spawn, forcing you to then sacrifice a unit.
People continually underestimate SS's difficulty, (on hard, obviously.) and while most if the game is piss easy, some chapers come to mind, such as 11 Ephraim, 25(?) either, (the defence one.) and the chapter against Vigarde, are pretty hard. The fact that you can grind kicks the difficulty down to zero if you take advantage of it, though.
Anyway, sprites. They're basically like the recruit classes from FE8, but not children. Thinking about it, ignore the bottom rightmost one, and the valkyrie edit's legs. I, personally like the farmers, (they're the ones with hoes,) and the female librarian. (bottom leftmost.) I went back and fixed some of them up.