Spurs welcome the Championship leaders to White Hart Lane tonight with a place in the fourth round of the competition at stake.
Following an indifferent start to the Premier League season for Tottenham, Townsend believes that the hosts will need to be fully focused if they want to get past Forest.
"Our opponents are flying high in the Championship so we know it's not going to be an easy game. They're going to try and get at us from the first minute and it's going to be difficult, but we're playing at home in front of our own fans so it's definitely a winnable game for us," he told the club's official website.
"You've always got to be more wary of teams when you play in the cup competitions because they're going to be up for it that little bit extra and they're going to be running around that little bit more, so we've got to be on our game if we want to go through."
Spurs head into the match on the back of a 1-0 defeat at home to West Bromwich Albion in the league.