The Reds finished second in the league behind champions Manchester City, despite accumulating a staggering 97 points.
And Castles thinks Jurgen Klopp can take his side even further next time out with the addition of more firepower up front and increased creativity in midfield.
“If you look at their squad, it’s very good but it’s not as deep as their rivals,” he said in The Transfer Podcast.
“They’ve been fortunate in that they’ve managed to keep their front three fit for almost the entirety of the season. It’s only in these last few weeks that we’ve seen Roberto Firmino missing for a sustained period and obviously losing Mo Salah for the Champions League semi-final return.
“Their attackers, it’s about those three and it’s about the two full-backs. That’s where Liverpool’s goals come from.
“Andy Robertson’s been fit all season, Trent Alexander-Arnold’s been fit most of the season. They have got cover for Alexander-Arnold in Joe Gomez, but Joe Gomez they prefer to play at centre-back and is not as good as an attacking force as Alexander-Arnold. They don’t have any cover for Andy Robertson. They obviously need to buy there.”