Fort Slava was a fortress of the Mid-East Allied Forces near the coast of the continental mainland. After the Marley Mid-East War, Marley annexed the fortress and the naval port where the capital fleet of the Alliance was stationed. It was heavily fortified with wall mounted mortars and machine guns, as well as the new anti-Titan artillery.
Story: In the year 854, the weakened Allied Forces make their final stand at Fort Slava, a key tactical position protecting the Allied flagship fleet, stationed within the nearby naval-station. Marleyan forces wanted to take the area, knowing that attacking the port would be impossible without control of the Fort. The fortress held an armored train with anti-Titan artillery equipped, however, meaning that Marley would not risk deploying its Warrior forces without risk of defeat as well as severe casualties.
Trivia: The katakana spelling of Slava could be an anagram of "Basura" (バスラ), which is the Japanese name of , the second highest-populated Iraqi city. The fact that Slava faces a port could also be an allusion to Basra, which is located where the and the merge. Slava is a word that appears in numerous Slavic languages, and translates as "glory" or "fame".
Source: Fort Slava (Anime), available under CC BY-SA 3.0.