In the first game of a European double-header, Treviso struggled to contain Northampton as they ran in six tries at the Stadio Monigo.
The Italian side sit bottom of the GUINNESS PRO12 and are still searching for their first win of the season, but after a 24-24 draw with three-time European champions Leinster and last weekend's performance, Casellato is satisfied his side are moving in the right direction.
"What is positive is the fact that we have conceded only one score from open play and that the total defensive plan worked," he told La Tribuna di Treviso.
"We suffered on defensive drives from the lineout and scrum and Northampton put us under extreme pressure, especially at the beginning of the second half.
"But the defence remained strong, and the problem to be solved is instead at the forefront. So we must get our heads down and work to grow.
"I can not say anything more to the players. They have given everything facing the best defence and best attack in the English Premiership.
"We tried our best. That Northampton team is very strong but the performance was good and the team has made further progress."
One bright spark for Treviso was the performance of fly-half Joe Carlisle, who scored one try from an interception and set-up another, and Casellato is thrilled with the 27-year-old's improvement since his summer move from Wasps.
He said: "His performance was good. Sam Christie gives him a hand with the language and he speaks more Italian now, so is able to be more of a leader and have more dialogue with peers.
"The feeling leaving the field is that we can hold our heads high having given the best of ourselves to honour the jersey. We earned their respect.
"We know that the next match at their ground will be even more complicated, but we will have to make their lives difficult."
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