Castle began his career at Bournemouth aged nine before moving to Readingin 2001 where he broke into the reserve team at 15. He made his Reading debut as a 79th-minute substitute against Watford on 30 April 2003 along with fellow Academy graduate Darren Campbell. At just 16 years and 49 days he became the club's youngest ever player, a record he still holds. Despite making his debut at such a young age, he could not establish himself at Reading and the match against Watford proved to be his first and last appearance for the club. In September 2005 he moved to St. Albans City on a one-month loan and made six appearances in total before returning to Reading on 11 October. Soon afterwards, Castle was soon loaned out again, this time joining Rushden & Diamonds in January on loan for the rest of the season. He made his debut against Wrexham, starting in a 2–0 home defeat and with his Reading contract due to expire at the end of the season, Castle made his loan move to Rushden permanent on the last day of the transfer window, signing on a short term deal until the end of the season. He made one further appearance on 24 April in the Maunsell Cup against Peterborough United before being released by the club at the end of the 2005–06 season.
Non-league
Following his release Castle moved into non-league football. He joined Staines Town in July 2006 but remained there for only a few months, making nine league appearances, before joining Conference South side Eastleigh in October. He made just one first team appearance for Eastleigh, a league match against Weston Super Mare, before moving on to Bashley in the Southern League Division One South & West a month later. Castle remained at Bashley for three-and-a-half years and helped the club to promotion to the Southern League Premier Division during the 2006–07 season. He made 131 appearances in total, scoring nine goals before moving on to A.F.C. Totton in January 2010. After a brief stay with Totton, Castle joined Southern League Division One South & West rivals Gosport Borough in June, remaining there for one season before switching to Lymington Town on 1 July 2011. Castle was released by the club just three months later though when it emerged he had been playing Sunday league footballwithout permission. Following his release he joined GE Hamble until the end of the 2011–12 season before returning to Southern League football with Winchester City in June 2012. After early exits from the FA Cup and FA Trophy, Castle left Winchester and joined Sholing in September.
International career
While at Reading, Castle represented the England under-16s and the Ireland under-18s at international level.
