Tottenham Hotspur reportedly "went a long way down the line" with Sevilla boss Julen Lopetegui before the deal fell through at the final moment.
"They went a long way down the line with Julien Lopetegui the Sevilla coach. They had agreed terms with Sevilla to release him from his contract for £4m, the release clause in his contract and also agreed terms on a contract," Castles told the Transfer Window podcast.
"Then for personal reasons related to family matters and because of changes brought about on movement of individuals because of Brexit, Lopetegui pulled out of that deal and Tottenham then went for Fonseca."
Lopetegui, who has previously been in charge of Spain and Real Madrid, joined Sevilla in 2019 and led the Spanish outfit to the 2019-20 Europa League title.