FrogReaver
As long as i get to be the frog
An abjuration wizard with a single level dip into cleric or fighter can cast a flaming sphere and then take the dodge action each turn. The flaming sphere in a tight space is going to wreck team enemy and all enemies will have disadvantage against the wizard, who will have as much hp and abjuration shield as a typical fighter has HP, as much AC as a shield using fighter and can cast shield when needed.I guess we run fairly different styles of games. Even in dungeons, I often have the hallways interconnect. The denizens know the tunnels while (unless they've already explored the area) the PCs don't. So turtling like that is a good way to have your back line flanked.
But yeah, it can work if the terrain allows for it. More often than not, in my campaigns, it does not, because I generally like to set up my encounter areas to encourage the characters to move around. To me, standing in a doorway slugging it out with a monster while the party attacks from behind is about the most boring form a combat can take. YMMV
I think a wizard focused on defense likely makes a better doorway/hallway blocker than the fighter, while providing an absolutely devasting effect for team enemy to deal with (and higher level wizards only tend to get better spells for that situation).