You need to make a quality decision on that stairway:
Do I want to harden the basement so that I can seriously believe no one can get down there without sawing bars, . . . breaking locks, . . . etc.?
Do I want to just take the chance and run pall mall down the stairs with the idea that I might get the jump on them?
Do I want to make it easy for me to get to the basement un-detected and ready if need be for a fight?
If you answered yes to either of the first two, . . . you know what you have to do.
If you answered yes to the last one, . . . get a couple of your strong back (and weak mind) buddies, . . . and build two things: first a new set of concrete steps (they don't creak or give when you step on them), . . . and lay up a 4 inch solid concrete block wall on the open side of your stairs, . . . all the way to the bottom.
The total job if you are handy at all yourself shouldn't cost more than a couple hundred bucks, . . . and will put you all the way to the bottom, safely.
May God bless,
PS: Want to guess who does not have a basement in their house, . . . and for what reason, among others