Costlemark Tower is one of the toughest dungeons in Final Fantasy 15, combining a lengthy multi-floored dungeon topped off with some tough bosses, including the Red Giant and Jabberwock.
Make sure you're a high level, have a good meal inside you and have some EXP buffs to get the most of its spoilers. Also, we'd recommend you follow Dino's questline before tackling this one as the final item he needs is in here and it will save you a second visit.
When you're done, be sure to check out other Final Fantasy 15 dungeons and tombs for more places to explore.
Costlemark Tower for Tomb of the Tall
Recommended level: 55
Head for Fallgrove and walk east up the path to find the tower, but bear in mind that the doors don't open during the day so you need to be there between 7PM and 4AM.
Head inside and follow the path around, and in the first room you'll encounter some Gelatins, tougher cousins of the Flans. Head west from the room and follow the path round until you find more Gelatins and some Galvanades - these guys can be a real pain as they have massive Lightning AoE attacks, so focus on them first if you can.
Keep following the path until you reach a room with two collapsed balconies; cross the narrow beam on the left and descend the stairs to the next floor.
You'll now be descending a couple of long spiral ramps - there are more pockets of enemies along the way, but you'll eventually reach the bottom. Head through the exit in the north wall, crawl through the small gap, and turn left into the corridor. (If you're following Dino's stone location quest, the Emerald Stone will be in this room.)
Head down the stairs at the end and keep following the passageway through a couple of open rooms full of Gelatins and Galvanades until you reach a long curved ramp. Follow it to the bottom and you'll emerge in a large rectangular room. On the floor there are some glowing blocks, and there's also a red switch on the floor. Don't stand on that as it'll send you back to the entrance, and instead stand on the glowing block in the south west corner of the room and activate it to descend to a labyrinth below.
You'll now find yourself in a series of small rooms each full of enemies that you'll need to kill to proceed. Activate the blocks in turn to progress through the maze, and when you reach a room where you have a choice choose the one facing east - heading south will lead you to the Red Giant sub-boss and force you to double back.
(If you're looking for the locked door after gaining access to them, you'll have to face him - it's through a small hole in the wall on the north side of the room. Be aware he's resistant to everything apart from your Armiger and Royal weapons.)
When you finally emerge you'll be in a small circular room. Heal up and get ready to fight, then descend the elevator to face the Jabberwock. His main attacks are a tail sweep and stamping his massive feet, but he'll also cause Petrification if you get caught in his breath - you're immune to damage while you're turned to stone so it's not a major problem, but it's still best avoided.
Need more help? Our Final Fantasy 15 guide and walkthrough can provide tips on main story, as well as the open-world's many quests and activities. Learning how to get AP fast, EXP fast and money or Gil fast will aid you in many optional dungeons and tombs - including how to open and explore their locked doors. There's also plenty of interesting side-quests, too, such as the Scraps of Mystery and Professors Protege frog locations. And if you want to get around easier, you can rent a Chocobo, learn the infinite sprint trick and later in the game, get the flying car Regalia Type F. There's also more to see and do with DLC, such as Episode Gladiolus and Episode Prompto.
As this is a boss his weaknesses and resistances are hard to gauge, but we found a combination of Warp Strikes with a Broadsword together with Gladio's Tempest Link Attack worked fairly effectively.
Once the beast is vanquished use the red switch on the ground to return to the entrance, and if it's now daytime you'll need to use the white switch by the entrance to get back outside, and don't forget to return to Dino if you grabbed the stone for him.