From a game meta p.o.v. the dog (or any animal or even a depowered turret) behind a door causes collision which means the enemies line up single file and wait patiently to enter the serpentine tunnel instead of bunching up. This video from AdamVsEverything explains it: https://www.youtube.com/watch?v=eTJf0-pusxQ

I like using a guard dog in a kennel as it doesn't break my role-play immersion like an unpowered turret would, and lines up with the pre-gunpowder forts I saw in West and South Asia that would have a guard dog kennel at the entrance.

Cool! What's the purpose of the door the opens onto a wall near the end of the trap tunnel where it opens onto the killbox?

Second playthrough & build. Vaguely 40K Imperial Guard themed, no power armour, no genemods, only the bionics needed for biome survival and replacement missing limbs. Use heavy artillery from an entrenched position to bombard the enemy then lure survivors into the killbox where our less powerful shorter-range weapons (chain shotguns & heavy SMGs) had an advantage. Living in the desert/shattered biome of their homeworld like the Tallarn Raiders or Kreig.

First playthrough & build. Vaguely 40K Astartes theme with heavily genemodded heavily bioniced marines who would engage enemies in the field, though usually at distance due to their weapons range (miniguns on regular marines, charge lances on double passion shooters, zeushammers on cataphracts). Living in the forest with many trained wargs & animals à la the Space Wolves or Caliban-era Dark Angels.