Castagne made his debut for Genk, after progressing through the Belgian club's academy system, during a 1-1 draw with Club Brugge for former Rangers and Aston Villa manager Alex McLeish's men on Sunday 14 August, 2014 before going on to appear on 27 occasions that season.
His impressive maiden season in Belgium's top tier led to Castagne earning a call-up for his country's Under-21s side later that term, starring alongside future Foxes team-mates Youri Tielemans (bottom row, second from the left) and Dennis Praet (bottom row, far right).
The European stage
Another 21 appearances followed for the talented full-back in 2014/15 before, a year later, he represented Blauw-Wit in the UEFA Europa League.
Finding the net
Castagne began to develop a talent for scoring big goals for Genk and, on Thursday 16 February, 2017, he found the net again in a UEFA Europa League Round of 32 clash with Astra Giurgiu, eventually helping to seal their place in the next round.
Into the latter stages
His goal in the second leg of the next round's tie with Belgian rivals Gent on Thursday 16 March, 2017, on the other hand, contributed to a 6-3 aggregate success. Genk, though, would eventually lose out to Celta Vigo in the quarter-finals.
Impressing for Atalanta
In the summer of 2017, the Belgian made the switch to Italy to join Atalanta and immediately helped I Nerazzurri continue their transformation from regular relegation battlers to serious contenders in Serie A. He also starred for the club in the UEFA Europa League, narrowly losing out to Borussia Dortmund in the first knockout round.
Making his Belgium bow
On Friday 7 September, 2018, Castagne's form in Italy was rewarded when he made his senior debut for Belgium in a routine 4-0 triumph over Scotland at Hampden Park, Glasgow.
Challenging at the top
Atalanta finished third in Serie A in 2018/19 - booking a place in the UEFA Champions League - as Gian Piero Gasperini's side secured several memorable results, including a 3-0 success over Juventus on Wednesday 30 January, 2019 in which Castagne scored his side's opener at Atleti Azzurri d'Italia.
Linking up with Tielemans
Later that season, on Thursday 21 March, 2019, he continued to play an integral role for Roberto Martínez's Belgium, laying on an assist for Youri Tielemans in a 3-1 victory over Russia in Brussels.
Scoring for the Red Devils
Castagne got off the mark in the international scoring stakes by finding the net a 2-0 win against Kazakhstan at King Baudouin Stadium, Brussels on Saturday 8 June, 2019. He would later add another to his tally during a 9-0 rout over San Marino.
Starring in Leicester
The Arlon-born full-back sampled a King Power Stadium matchday in the summer of 2019 as Atalanta faced Leicester City in a pre-season friendly, narrowly losing out to the Foxes 2-1.
Signing off in Italy
The 24-year-old enjoyed arguably his best season for Atalanta in 2019/20 as the Italian outfit once again finished third and reached the quarter-finals of the UEFA Champions League, losing out to Paris Saint-Germain. Castagne made 35 outings in all competitions for Gian Piero Gasperini's men, netting two goals.