The Time Team archaeologist said areas surrounding the likes of Oswestry's Iron Age hillfort – which he described as "spectacular" – were just as important as the hills due to their historical and religious significance.
He made the comments while attending a seminar event at Oswestry Memorial Hall, which was held by campaigners fighting plans to build 117 homes near the town's hillfort.
More than 100 people attended to hear a number of expert speakers from across Europe discuss the the context of the hillfort, its historical significance and some of the archaeological finds made at the site.
The homes off Whittington Road have been included in Shropshire Council's Site Allocations and Management of Development (SAMDev) plan, which will see more than 20,000 homes built across the county by 2026. Two further proposals to build homes near the hillfort were last week omitted from the planning blueprint, which had initially proposed about 200 homes would be built in the area.
Mr Ainsworth, a regular on the Channel 4 programme, said: "This is a spectacular hillfort. One of the impressive things about it is there are some unusual features which we don't quite understand, which makes it unique and really quite unusual."
"It's important that we treasure the past. The zones around the hillfort, the penumbra, are just as important as the hill. Even in prehistory these areas had meaning for religion and history."
Mr Ainsworth, who lives in Chester, has been studying Iron Age hillforts for 40 years and said he had long had an interest in the hillfort in Oswestry. "I've got a professional and personal interest in any development which potentially affects a major Iron Age centre," he said.
Among the speakers at Saturday'sevent was Dr George Nash, professor of archaeology and anthropology at IPT in Portugal. He said: "Judging by the audience that turned out, clearly there's an opposition against this ridiculous planning proposal. We've got to keep our green and pleasant land free of development for future generations."
Maggie Rowlands, one of the organisers of the seminar, said: "We had a fantastic turnout. "
Town councillors are due to meet tomorrow to form a response to the proposals.