My 2 cents.

Your fine with enforcing a setting or rough idea like a male Knight, older male charceter, or other small hints to keep direction and tone. 

But other than the base points. How they want to shape that idea is up to the player. You can maybe set a charecetistic or so you want, or range of those, fitting setting etc. But after that nope. 

Ie. 

A female lady/princess , unmarried, youngish. Grew up mostly within thr Palace. 

Now plenty of room. Detailed. Ideal for a set story advanced or 1x1

A female charceter. Younger x older male chacetwcer. Some hidden aspect to past. 

More vague. It's just a sketch of thr aim. There's restrictions but there not strict. 

Not a set charceter, they just in that case may have to play a role. How and what they bring to it is there own.