It will be Marshall's first season in the top-flight.
"It's going to take a bit of getting used to," he told the Western Mail.
"I'm expecting a whole new challenge and I'm preparing for that now.
"At Championship level there were always a lot of crosses coming into the box, but I don't expect the same number in the Premier League.
"The best teams will look to play their way through rather than fire it into the penalty area early.
"I'll find that out for myself over the next few months but I have spoken to goalkeepers who have played in the Premier League."