Block stairway entrance

Hello,

we want to block off the staircase going upstairs because there are cats there that are not allowed to come down. No worries, the space upstairs is big enough for the cats. :-) Pictures of the staircase are attached, as well as the measurements. What options do I have to block off the stairway so that people can still pass through? Maybe a door? How can it be attached there? And how do I close the small triangle on the side?

Best regards

Build walls and a sturdy door, nothing else helps. Our fat tomcat even squeaks through the bars of the stair gate ;).

But why do you actually want to lock it off?

The house cats are not allowed downstairs. We, as young adult students, also want a bit more privacy upstairs. :-)

Can the triangle on the side be closed off with drywall? If yes, how should I best start? And how do I proceed then?

Wall + door, as already said by Uwe. How does it look up there? Is it easier there to block the way for the cats so that they can no longer come down?

From above, it is even more open.

The house cats are not allowed downstairs. Also, as young adult students, we want a bit more privacy upstairs. :)

Do you live there as tenants?
What does the landlord/owner think about possible renovation measures? Everything looks so new to me...