They are often joined by other rough sleepers who bed down only yards from the tourist trail to the castle gates and the statue of Robin Hood.
The bowels of Nottingham are criss-crossed by a warren of man-made caves, many of which were used as dwellings and cellars in medieval times and later as air-raid shelters. Now they have become known as a communal alternative to street doorways, and about half of the 30 to 40 people who sleep rough in the castle area now live in the caves.
The city council, concerned at the "health hazard" and the effect on tourism, has already blocked some of the caves with metal grilles and intends to close off all of them soon. But the inhabitants say they will move on to other caves or climb over the grilles.
One or two rough sleepers had used the caves in previous years but it was not until this summer, when about 30 began to sleep in two caves under the castle gatehouse, that the council took action. The caves were hosed down, to clear the litter that had accumulated outside, and blocked off, with many of the inhabitants losing their belongings in the process.
"We don't want Nottingham to turn into a cave-dwelling city," said the council leader, Graham Chapman. "It is insanitary and it could become a health hazard. We have worked very hard to provide hostels and there is no necessity for people to live in caves."
His comments are disputed by Tas and Jo, who say they do live cleanly and take out their rubbish and whose cardboard "carpet" was removed by the council last week. However, some of the longer-term residents admit to becoming concerned: "No one knew about the caves before and we had one to ourselves," said Caroline Golding, 19. "Now you get about eight to 10 people in one cave, and it gets very crowded when it's raining."
For many of these people, the prospect of finding alternative accommodation is not as straightforward as Mr Chapman suggests. The caves are an improvement from doorsteps and for Jo and Tas the choice was simple: they either stayed with each other and their dogs or risked being separated by going into hostels.
They are both unwell. Jo, 21, is an alcoholic and has stomach ulcers and hepatitis C. She left home at 14 after falling out with her father and has been sleeping rough on and off ever since, often with only a dog for company and security. Tas, 29, ran away from a care home at 14 and now fears he has terminal cancer.
"We are trying to get a home but they don't allow dogs," says Jo. "The dogs are our children, they are with us all the time, that's why they are so friendly. They keep the rats and the weirdos away."
The bonds that people such as Jo and Tas develop with their pets is being addressed by the Macedon Trust, Nottingham's biggest provider of accommodation for the single homeless. Its research found that eight out of 10 rough sleepers had pets - and most refused to use night shelters because of a ban on pets.