“I THINK everyone has this special place in their brain — a primal nostalgia — for a treehouse,” said Alexandra Meyn, bundled up in a sweater on a recent afternoon inside the airy perch she built behind her garden apartment in Brooklyn. “People’s eyes light up when you tell them you have a treehouse.”

Secured to a solid old mulberry, Ms. Meyn’s treehouse conjures up childhood only if you were a really cool kid. The interior is covered in a collage made from the pages of fashion magazines; an electrical cord that stretches from her bedroom powers a string of lights and a record player. There also are pink bats on the wall, and glass windows that dangle on the ground-floor level like earrings.

When she completed the house six weeks ago, Ms. Meyn invited friends over for a masquerade party. “It’s fun to have people up here and talk, talk, talk,” she said.