The Tigers, who finished sixth in the 2016 regular season, had the second best attack after Warrington (675) and scored 617 points.
But Castleford’s defence was one of the worst in the competition, with only Wakefield leaking more points after 23 rounds.
Millington says his side have changed their pre-season drills up to help try and create a meaner defence.
“We’ve done a lot of our running in games, which you’re still getting the miles and that in your legs but you don’t think about it as much so it’s been quite enjoyable,” the prop told Love Rugby League.
“The way we’ve been doing the running in the games it puts a lot more ownership on you as well because you have to communicate in the games when you’re tired, which is what let us down at times last year.
“Our defence and our communication within our structure we’ve seen that improve a lot already in the pre-season, which is what we need to improve.
“Our attack, we’ve still been working on it because it’s something we enjoy doing.
“Obviously you can see everyone enjoys attacking in our side and we all play a part in it but our defence and communication and defensive structures is something we’ve been working on a lot.
“In the Hull game we didn’t have a full side out but I thought we were good for most of the game defensively and hopefully well keep improving moving into round one.”
The Tigers head to Lanzarote next to continue their pre-season in warmer conditions.
The squad departs after their friendly with Wakefield this weekend.
“I cant wait to get over there and warm up a little bit, we went there last year and it was fantastic,” Millington said.
“We’ve got good chance for us to really fine-tune stuff we’ve been working on over here in the cold and get a good feeling and a buzz about the boys, which when we come back will really help us.”
Castleford have been close to cracking the top four in the past few seasons and Millington believes that has to be their goal in 2017.
“All we want to take is that next step and go for the top four,” the Australian said.
“The Challenge Cup, everyone wants to win that, but in the regular season that top four I think is the minimum we should be aiming for.
“We just missed out on the League Leader’s Shield a couple of years ago, we’ve had two fifths [place] and I think it’s time.
“We’ve been together long enough now and I think its time we take that next step and make the top four and push for a trophy as well.”