There are nearly as many girls here as boys. In charge is Lorraine New, who is used to fighting her corner in a macho world. One of her helpers is Emma Robinson, 18, who after a bruising life, much of it in children's homes, has ambitions to make it as a boxer. Other young women taking part are recovering self-esteem and a sense of purpose having once been cowed by abusive male partners.
The boxing project has changed their lives. Within three months some achieved basic fitness instructor certificates. Now they run sessions for others from the women's refuge where many were recruited.
"It's about learning, not about fighting," says Tracy Durdan, 29. "When you're in an abusive relationship, you're told you are crap. Just walking into a gym by myself is something that wouldn't have happened - Lorraine has given us the confidence to come here."
The women get fit and learn all sorts: diet; anatomy; first aid. "It's not so much about getting strength in my arms, it's the strength in my head."
says Ellen Murray, 23.
Emma's fast-developing talent means she needs male sparring partners. She's been boxing for three years, having met Lorraine when living in a bail hostel. Emma hopes to go to East Durham and Houghall College, a sports academy that specialises in boxing. "I come up here four days a week. If it wasn't for this gym I don't know where I would be," she said. "I don't drink as much as I used to. People thought I'd become a raving alcoholic or end up in jail."
Box also recuits from Northampton's rougher estates. Lorraine is quick to stress that she teaches discipline and self-control rather than exacerbating aggression and is clear about her role: "I'm a coach, not a social worker or a youth worker."
Numbers through the door bear witness to her success: 525 in 2003 and, so far, 1,379 this year.
But she badly needs funds to install a much-needed boxing ring so people can spar properly and ensure her gym's future. Subscriptions and pound;6,000 from the Local Network Fund have kept it going for two years but she is now bidding for cash from Sport England to enable her to employ two full-time staff.
Lorraine qualified as a boxing coach nine years ago. She ended up running her own gym after parting acrimoniously with the "male-dominated" club she belonged to. Lorraine, who has a BTec in mentoring, worked through the Millennium Fellowship and the Royal Philanthropic Society to recruit youngsters on bail to the gym. Then women from the refuge began coming after she spoke at a lifelong learning conference about getting women out of the house.
"Fifteen out of 18 came to the gym," she said. "We paid for the course that has led them to become qualified as gym instructors. Now they are volunteers here. There could be a real future for them.
"I do classes in fitness as part of lifelong learning, and there's a growing need to have adult instructors."