The Derren Litten sitcom's fifth series launch grabbed 6.24m (26.7%) for ITV1, adding 406k (2.3%) on +1.
However, despite blitzing the 9pm hour, ratings were down by over a million on last year's record-breaking opener.
BBC One's old episode of New Tricks was closest to the holiday comedy, drawing 3.96m (16.9%), while The Mentalist took 1.31m (5.6%) for Channel 5.
A Melvyn Bragg documentary on BBC Two, Class and Culture, interested 1.32m (5.7%), and 990k (4.2%) watched George Lamb's The Bank Job on Channel 4 (+1: 185k/1%).
At 10pm, Castle logged 1.02m (5.6%) for Channel 5, but was beaten in the hour overall by BBC Two, which had 2.03m (10.4%) for a QI repeat.
BBC Two's Mastermind held 1.83m (7.9%) across the 8pm hour for its double bill, while World's Toughest Trucker was seen by 1.11m (4.7%).
Channel 4's latest screening of New Girl appealed to 920k (3.9%) at 8.30pm (+1: 208k/0.9%), and 487k (2.7%) tuned in for The Mad Bad Ad Show at 10pm.
Overall, ITV1 cruised primetime with 24.9% (+1: 0.8%), beating BBC One's 20.4%. BBC Two claimed third place with 6%, ahead of Channel 5's 4.8%, and Channel 4's 3.8% (+1: 0.8%).
Ruth Jones comedy Stella continued on Sky1 with 570k (2.4%) at 9pm. The Big Bang Theory fetched 601k (3%) for E4 at 6.30pm.
BARB ratings data supplied by Attentional