Is this a difficult tour for people with mobility issues?

Yes. There are lots of stairs, uneven ground and small corridors.

Yes the tour goes through the basements of several old buildings and to enter and exit each involves going up and down stairs. There's also a little bit of a walk between the buildings on sidewalks and involves crossing streets several times.

It might, theres a lot of stairs

Yeah, you have to travel on a few flights of stairs to get down to the underground. There are no elevators or wheelchair accessible ramps. However the total distance you have to walk for this tour is not too long. It's 4-5 stair trips (4 times down and 4 times up) if I remember correctly.

Yes, stairs down and up. Uneven ground throughout.