“I don’t believe in fortresses.” – Magnus Carlsen
I understand Carlsen’s famous quote, not at face value, but rather as an understanding of chess’s nearly infinite possibilities. Carlsen himself has shown in a lot of his games and how seemingly easily drawn games can be won by finding hidden resources in the position.
Still, fortresses do exist in chess. Even Carlsen himself has stumbled upon one or two in his career. Perhaps the costliest one was in the fourth game of the match with Karjakin, when in a relatively easily won position he allowed Karjakin to build one. Later in the match, the missed chances of an early lead could have cost Carlsen dearly.