lair

the secret home of a fierce or wild creature — typically a hole, cave, or thicket where the animal sleeps and shelters between hunts — and, by extension, a tucked-away spot that a person uses as a private hideout, retreat, or base, often with a slightly dramatic or mysterious feel (a criminal's mountain hideaway, a villain's secret base, or a quiet attic room someone disappears into to work).

• den

  near-equivalent; 'den' is more everyday and also used of bears, foxes, lions

• hideout

  for people only — a place where someone hides from the law or from others

• burrow

  smaller, dug by the animal itself, used of rabbits, foxes, badgers

• retreat

  softer, positive — a quiet place to relax, not necessarily hidden