It's confirmed: PETS AND VEHICLES SOMEHOW COME FROM THE TOWER BEYOND FIREBIRD CASTLE.
However, there are a great many things to note about this area:
1) The monsters are high level and elite.
2) There is a level cap of...49? 45? Remind me to check.EDIT: It's 45 [Maxi]
3) There is an extremely tedious quest on each floor (at least the two floors we've been to!) that requires you to kill these monsters for quest drops. EDIT: Since all the floors are the same, this quest is found on all the floors.
4) There are at least 3 floors, and we've only been to the second one. EDIT: There are 7 floors, according to the map list in game.
5) There is a boss on each floor so far. EDIT: All the floors look to be identical, so it's safe to assume there's a boss on every floor.
That being said, there are some potential upsides to all of this:
1) Upon defeating the boss, you're rewarded with quite a bit of EXP and a box with nice items.
2) Monsters drop great items here.
3) Pets and vehicles somewhere, probably at the top!
Some precautions to take:
1) Fully repair your equipment, bring spares, and if you use ammunition or potions, bring lots of them.
2) Take a healer, you will die without one...and have to go up the entire tower again if you do. The revive skill negates this need.
3) Bring people who can stun enemies, as it's an extremely useful skill to have.
4) Have at least 2000 gold on you, so you can leave the tower.