[10] Though strong form placed him close to a senior call-up at stages, a late season injury would ensure he did not make his AFL debut in 2015.

[10] He was named as an emergency in the club's round 6 match against Port Adelaide, but was called to play when wingman Kamdyn McIntosh withdrew due to injury[12] He recorded 11 disposals on debut.

[19] Castagna, along with forward-line companions Daniel Rioli and Dan Butler, played a key role in Richmond's five game unbeaten start to the 2017 season.

[16] Castagna received the AFL Rising Star nomination for round 19 after kicking three goals and gathering 16 disposals in Richmond's 33-point win over the Gold Coast at Metricon Stadium.

[26] Castagna entered the 2018 season having earned a permanent promotion to Richmond's senior list and signing a new two-year deal with the club.

[36] He played the following week against West Coast but missed the club's round 10 match due to some later soreness that emerged as a result of that hyper-extension.

[39] During that match he received a nomination for Mark of the Year, with a fourth-quarter forward-line specky attracting the highest fan vote of three nominees that round.

[16] In the first week of the finals series the minor premiership winning Richmond hosted fourth placed Hawthorn where Castagna recorded seven tackles but failed to score himself in what was a 31-point victory.

[16] Castagna underwent a restricted pre-season training program in the lead up to the 2019 season, after suffering a minor ankle syndesmosis sprain in early January of that year.

[59] Castagna was prolific in the grand final, recording 20 disposals and six shots on goal in his side's 89-point victory that earned him a second premiership in three years.

[64] Castagna featured in the pre-season series in March 2020, and was named by AFL Media as among Richmond's best players with a two-goal performance in the last of those two matches.

[65][66] He kicked an equal team-high three goals in a round 1 win over Carlton when the season began a fortnight later, but under extraordinary conditions imposed on the league as a result of the rapid progression of the coronavirus pandemic into Australia.

[77][78] He kicked two goals and had seven marks in round 9's win over the Western Bulldogs, but one week later The Age labelled his output that season as "modest".

[79][80] Despite a five disposal game in round 11 that 1116 SEN labeled as having "almost zero impact", Castagna ranked equal-third for goals, third for marks inside 50 and equal-fifth for inside-50s among Richmond players to that point in the season.

[81][82] In round 14, Castagna was named by AFL Media as among Richmond's best players in a win over West Coast, after recording 16 disposals and kicking one goal.

[86][87][88] Castagna became a three-time premiership player in the grand final the following week, kicking a goal in a third-quarter fightback as Richmond defeated Geelong by 31 points.

[91][16] Castagna plays as a versatile small forward with good marking abilities for his height, reliable offensive skills and is also particularly adept at applying forward-half defensive pressure.