Speaking to Sky Sports News, Southgate hailed his team's performance following a gruelling penalty-shootout win over Colombia on Tuesday night.
"We are a team that are still improving, we know where we are and we are having success because everybody is working hard on the field," the manager said. "The collective has been key, all of the support staff, all of the players have been tight. To get through the two games we had this week, we needed all of that because we're not the finished article.
"We don't have renowned, world-class players yet but we've got lots of good young players who are showing on the young stage that they can be brave on the ball, try and play the right way, have shown some mental resilience.
"To come on the back of a game where we had to go to the very depths emotionally and physically in midweek and be able to give what we gave today - control the game, but also withstand the physical test - was a real sign of resilience for a young team, who I think are really maturing in front of our eyes."
Croatia will meet England in Wednesday's semi-final tie after overcoming Russia on penalties.