A new visitor centre and accommodation has been developed at Harlech Castle, and a new footbridge installed.
The Cadw scheme, funded by the Heritage Tourism Project, is expected to attract more tourists to the area.
The West Cheshire and North Wales Chamber of Commerce said it would create "robust" economic benefits.
Executive director Colin Brew said the development had "already had a positive impact in terms of opportunities for local businesses and boosting the local supply chain".
And he believes it will continue to boost the economy by attracting more visitors, in turn establishing itself as a "key visitor attraction" in north Wales.
His comments came as First Minister Carwyn Jones visited the site on Thursday.
"In 2014, the heritage economy in Gwynedd employed over 8,000 people — 15% of all employment in the county," Mr Jones said.
"It's therefore vital that we continually improve the heritage tourism offer to attract even more visitors."
A £5.9m redevelopment project at a Gwynedd  castle will bring a "wealth of future economic benefits" for local businesses, a trade expert has said.