Preventing elderly guests with poor vision from falling down stairs

My family has a summer house with a steep, poorly lit stairway to the basement. The top exits into the kitchen. There is a door that is normally kept closed at the top, and guests other than children rarely need to go down these stairs. So, what's the problem? Recently, a family friend with a similar setup had their elderly friend tumble down such stairs (presumably confusing it with the bathroom), and were badly injured and had to be rushed to the hospital. We have lots of elderly friends/family/neighbors that regularly come over. So... ...what solution would you suggest to prevent elderly guests with poor vision/balance/sense of direction from accidentally falling down these stairs?

I know a simple lock will not work, because other guests will inevitably find it a nuisance and leave it unlocked, defeating its purpose.