Since Diego Simeone became Atletico Madrid manager in 2011, the team has been renowned not only for it's grit and determination, but also for a steeliness at the back that makes them almost impossible to score against.
The 'Rojiblancos' have conceded the fewest goals in La Liga this season, shipping only 18 in 35 games so far and their ability to grind out a 1-0 win, something which often happens, has put them 2nd in La Liga.
Team mates for club and country (Uruguay), Diego Godin and Jose Gimenez's partnership in central defence has become the most solid in Spain. They put everything on the line and heroic, last-ditch defending has become a hallmark of this team, not just this season but in previous European campaigns.
Atletico are a team who have knocked out Barcelona and Bayern Munich from the Champions League in recent years, and they were one Sergio Ramos header in the 94th minute away from winning the competition in 2014. Two years later in Milan they took Real all the way to penalties. Godin in particular was central to these runs, even scoring in the first final.
In addition, when the two at the back are breached, there is still Jan Oblak to beat - as Arsenal will know all too well from the first leg. The 'keeper was in stunning form at the Emirates, making seven saves, but his statistics when he plays at home are out of this world.
In La Liga he has not conceded on his own turf since January 20th. In his 78 appearances at home for Atleti, he has 51 clean sheets.
This is the task facing Alexandre Lacazette and company - get through this impenetrable barrier while somehow stopping Diego Costa and Antoine Griezmann from battering their way through at the other end. No wonder Simeone left the Emirates last week with a grin on his face.