The window marked the end of an 18-month drought without a single signing for manager Mauricio Pochettino.
But the Lo Celso deal, one which Pochettino chased after all summer, has left Spurs’ fans questioning the reasoning behind a loan.
Castles said: “I have Tottenham down as the club who have spent the most, both gross and net, in the transfer window.
“Just marginally above Manchester United and Manchester City. The [Giovani] Lo Celso loan deal needs to be factored in, it’s a loan with an obligatory option to buy for €60million.
Details on Giovani Lo Celso's loan deal move to Tottenham revealed (Image: GETTY)
“Real Betis are briefing their reporters - the reason it was structured as an initial loan is to do Paris Saint-German out of a percentage profit that they’re due on his transfer as a result of the deal Betis did with PSG last season.
“They knew Tottenham and other clubs wanted to sign the player and the structure, they say, is being used to reduce the amount of FIFA mandated solidarity payments in which they have to pay to Lo Celso’s previous clubs.”
Castles went on to state, however, that the deal has left people in Spain questioning if what Betis are doing is legal.
He added: “It sounds tricky but I can tell you that people in Spain don’t believe it will work and are questioning the legality of it.
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Details on Giovani Lo Celso's loan deal move to Tottenham revealed (Image: GETTY)
“From Spurs’ point of view, it’s irrelevant, they get the player at the price they agreed with Betis and I think more importantly it goes some way to satisfying Mauricio Pochettino’s demands for a lot of new players this window.
“We told you he was looking for two central midfielders, a No 10 and two full-backs.
“What he got was Lo Celso, the No 10, Tanguy Ndombele, their midfielder. They didn’t get Bruno Fernandes as Tottenham refused to go any higher than their bid.”