On the top floor of the Hyrule Castle 1F there is Royal Broadsword, and when you go on top of the roof there is Royal Halberd. On the way there you can get weapons as well so it is recommended.
Collect Other Weapons As Well
Inside the Hyrule Castle, there are Royal Guard, Royal series, Dusk Bow and others, so it has a variety of items. By unlocking the nearby Shrine and using Ascend, you can get several equipments without fighting enemies.
Hyrule Castle Map
Available From Caves
Cape Cales Cliffbase Cave
North Akkala Beach Cave
Komo Shoreline Cave
Ulria Grotto East Cave
Isle of Rabac Gallery
Tanagar Canyon East Cave
Icefall Foothills Cave
Corta Lake Cave
Rodai Lakefront Tunnel
Calora Lake Cave
Where To Get The Pristine Royal Broadsword
The Shadow Soldier Which Drops The Royal Broadsword
Available From Shadow Soldiers When Decayed Weapons Are Broken
The Pristine Royal Broadsword can be obtained from the Shadow Soldier in the Depths. Note that the Undecayed Weapon will not appear unless you destroy the Decayed Weapon of the same name, which can be obtained on the Surface, so when obtaining the weapon, destroy it first.
Available Whenever A Red Moon Occurs
Shadow Soldiers respawn during the Red Moon, allowing you to obtain the Pristine Weapons repeatedly.