The Metius Observatory is a location within Infinity Blade 3, introduced in the Soul Hunter update in 2013. The Observatory itself isn't accessible outright, however, the ancient part is accessible at first in order to fight Oslim, the Deathless Bloodmage armed with the Infinity Swords.
The actual observatory can be seen in the introduction to the interlude and when fighting the first Titan. Equipping Siris with the Axe of Heaven and Isa with the Blade of Heaven, with the Map of Heaven introduced in the Kingdom Come update, the Observatory becomes accessible, with the telescope pointing at one direction. Here, Varun, the Watcher of Heaven, can be faced. The treasure chest in the observatory contains The Origin, the highest leveled armor obtainable without the Blacksmith's help. After facing Varun, the protagonists sees why the telescope was pointing at one particular direction: Heaven, the planet's moon.
The Observatory has two separate locations, presumably both are connected with each other until the Worker of Secrets and Ausar the Vile destroyed the planet multiple times, in which the Observatory is left to it's fate. Before being betrayed by Ausar, the Worker of Secrets sent Varun to the main building of the Observatory to observe Heaven's progress, presumably why it was locked in the first place. After the Worker was imprisoned in the Vault of Tears and Ausar slain in the Plains of Koroth by Raidriar, The Observatory was left on it's own.
Years later, The Worker was released by Ausar, now known as Siris, and proceeded to create the Ark, to burn the planet again and move to Heaven. To occupy Siris and Isa, the Worker sent Oslim to the accessible part of the Observatory to defeat both Siris and Isa and to this end, Oslim was armed with the Infinity Swords;however, Siris kills Oslim and gains the Infinity Swords.
After Siris defeated the Worker and activated the Redeemer on him, the Ark still managed to reach Heaven without burning the planet. Varun saw this and wondered why the planet still hasn't burned, which sees him concluding that Ausar defeated the Worker. Attempting to find out where the Ark went, Siris faces Lelindre to know where the Ark went. Siris defeated Lelindre, and with her dying breath, showed the Map of Heaven to Siris. It is here that Siris and Isa gained access to the original building, and met Varun. Varun tried to convince Siris/Ausar that they can still ascend, as that didn't succeed, Varun faced Siris and was defeated. Siris then sees why the telescope within the Observatory was pointed at a particular direction and sees Heaven, the planet's moon.