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Just like many open-world games, you'll earn more of your map of Hyrule by visiting Towers and activating them with the Purah Pad. Tears Of The Kingdom has a lot of Towers to visit and solve the puzzles of, and a particularly tricky one is the Sahasra Slope Skyview Tower.
Get ready to navigate a cave system, and don't expect that to be your only challenge, as there will be enemies and a final room that expects you to remember every power at Link's deposal.
Finding Sahasra Slope
Sahasra Skyview Slope Tower will be at the top of a hill just east of Central Hyrule. The name of the region you'll need to enter and that you'll earn the map for by completing this Tower is named West Neculda. There is also an East Neculda, so try and be careful which region you are in a while looking for the Tower, but you should be able to find it easily enough just by following the image above. The coordinates are: 1333, -1190, 0131
Speaking With Billson
At the front door of the Tower, you'll find a guard named Billson. He's about as stuck as you are when it comes to activating the Tower because, for some reason, the door refuses to open even though it's not locked. It can't be locked as it's a sliding door. Billson will correct himself when he mentions the wrong way to open the door; he was briefly confused as, in his own words, he's hungry. In particular, he's craving the mushrooms that grow in the cave system directly under the Tower.
The mushrooms are a red herring, but the cave system is still where you need to head next.
Sahasra Slope Cave
There are a couple of entrances into the cave just under the Tower. You can take whichever entrance you want; they aren't the important bit. Also, be warned there is a battle Talus with Bokoblins circling the Tower. They make a formidable foe, and some of the Bokoblins have bows and arrows, so you may just want to defeat them before trying anything else. You would be safe from them inside the cave, but they still pose enough of a threat that dealing with them is a good idea to recommend.
The most immediate problem with the cave is the rock walls that completely block your path. There will be a rock on the floor, but before the walls, so combine it with a held weapon, as rock-based weapons can break through these rock walls. There are enough rock walls in your path that you may end up breaking your weapon before getting far enough, but don't worry, more rocks, as well as claymores, will start to drop as you break more walls. The claymore can break a rock wall without needing to be fused with a rock or boulder, but you can still use the fused ability to add durability if you so choose.
There's a room with enemies and a treasure chest, but that's not the right room. What you're looking for is a room with a boulder and two rocks blocking an additional room. Move that boulder to find a secret spot with minable ore and a Bubbulfrog. Believe it or not, that secret room has nothing to do with unlocking the Tower. To do that, you'll want to stay just in front of the boulder.
Using Ascend
You will be directly under the inside of the Tower, so use Ascend on this spot on the ceiling. If you are misaligned, the grid will stay red, so aim for the round spot seen in the image.
Once you exit from your Ascend, you can walk over to the doors and attempt to open them from the inside. Turns out, that wasn't the issue, as they still won't open. Billson can see you from his side, and after the initial startle, he'll ask you if you can see what might be blocking the doors on your side.
Taking The Long Sticks
On both the left and right sides of the sliding doors, you'll find the Long Stick weapons jammed diagonally to keep the doors in place. Technically, there are a couple of ways to deal with this problem. The obvious solution is to just take both weapons, but your inventory for melee weapons might already be full. If that's the case, remember you can fuse these Long Sticks into any shield or weapon you are already carrying. If you can't do that either, you could just toss a weapon temporarily and take it back after you grab the Sticks but don't forget you can also just grab both of them using the Ultrahand ability and toss them aside.
Open the doors after, and Billson will repair the Tower. Use your Parah Pad at the terminal and be flung into the sky to survey the area. You will now have that map for West Necluda.