Meanwhile, midfielder Anthony Miles has also committed to a new deal with the club.
Castagna, who played all 25 games for Richmond this season, has penned a two-year contract extension, while Townsend has also been rewarded for his incredible form late in the year.
Townsend only returned to the team in Round 22, but kicked 16 goals from the final five games of the season to earn himself a one-year contract extension with the Tigers.
Miles, who managed only five games for the year and couldn’t crack the side during its successful finals campaign, has also signed a one-year extension.
“We are really pleased with Jason’s ongoing development and his workrate, both on and off the track,” Richmond footy boss Dan Richardson said in a statement.
“His pressure around the ball is a highlight and his brand of footy was very much aligned to the team success this year.
“Jacob played a significant role for the team in all our finals games, none more so than the Grand Final. He is a strong competitor and, having found his niche up forward, we expect he will continue to build on this in 2018 and beyond.
“Anthony is highly regarded by his teammates and the club as a whole. His strong work ethic, contested ball-winning ability and commitment to the team are second to none.