Born in Kingston upon Thames, where he lived only 15 minutes away from the Cobham Training Centre, Castledine started his career with Chelsea, who he had supported growing up.

[4] A year later, in March 2020, he rejoined Chelsea, naming Mason Mount and Jack Grealish as his favourite current players.

[3][6] Castledine made his senior debut for Chelsea as a substitute in a 6–1 EFL Cup semi-final second-leg win over Middlesbrough on 23 January 2024.

[7] On 26 August 2024, Castledine joined League One side Shrewsbury Town on a season-long loan deal.

[9] He is the son of former Wimbledon footballer, and television presenter, Stewart Castledine, as well as former Homes Under the Hammer host Lucy Alexander.