Hill is a small city-state located in close proximity of Roshtaria. Balta Hill is
a nation that probably has, or rather, had a long and storied cultural history.
Unfortunately, most likely, Balta Hill will only be remembered for two reasons. The first is that Balta Hill was the first city to bear witness to the resurrection of Ifurita. Shortly after she was revived, she leveled the city with several blasts comparable to a hydrogen bomb.
The second is that after the devastation, the city fell to Bugrom control, who in turn used it as a central base of operations for the Roshtaria Invasion. Jinnai had his own private base built near the ruins of the former city from where he gives orders to both his forces and Ifurita.
While one cannot be exactly positive of Balta Hill's exact location, we can make a reasonable guess based on several contextual clues. First, we know that it is a nation that neighbors Roshtaria. Shayla makes this clear when she grows angry over the Bugrom building a base in "Roshtaria's backyard." Second, we can see that the base is built on the side of a small mountain/cliff area. Also, it is one of the first places reached by Ifurita after Jinnai orders her to destroy the Alliance. This leads me to believe that Balta Hill is located on the small mountain range just south of Roshtaria and slightly to the east of Doros Land (this accepting the fact that the Forbidden Island is hidden somewhere in the wider area of the River of God.) This is all purely speculative of course, but it seems logical.