<?php

$array = [
"Apartamento" => [ "Characteristics" => [
 "external_area"=>
    [
      0=> "varanda",
      1=> "lavanderia",
      2=> "piscina",
      3=> "muros",
      4=> "area_serviço"
    ],
    "bedroom"=>
    [
      0=> "piso_frio",
      1=> "alvenaria",
      2=> "azulejo_teto",
      3=> "massa_corrida"
    ]
   ]
   ],
"Casa" => [ "Characteristics" => [

 "internal_area"=>
     [
      0=> "mesa de madeira",
      1=> "area"
    ],
    "options"=>
     [
      0=> "2 andares",
      1=> "banheiro externo",
      ]
   ]
   ]];
   
   
array_walk_recursive($array, function ($item, $key) 
{
    global $Characeristics;
    if (! is_array( $item ) ) 
    	$Characeristics[] = $item;
});

$result = array( "Characeristics" => $Characeristics );

var_dump($result);