From https://metaculus.com/questions/15849/ukrainian-control-of-mariupol-jan-1-2024/

Mariupol is a city in southeastern Ukraine located on the north coast of the Sea of Azov, which is currently under Russian occupation. Before the Russian 2022 invasion of Ukraine it had a population of over 400,000 residents, and it was an important industrial center in Ukraine. The city came under siege of Russian forces shortly after the start of the invasion. The Ukrainian organized defence lasted until 20 May 2022 when defenders were ordered to cease fighting. Mariupol is strategically important to Ukraine because it is a major port city and serves as a gateway to the Sea of Azov. It also has a huge symbolic value because the defence of Mariupol by Ukrainian forces, including their heroic defence of Azovstal, has become a symbol of Ukrainian resistance against Russian aggression.

Creative Commons Attribution-ShareAlike 2.0 license (CC-BY-SA 2.0) Lencer, CC BY-SA 3.0, via Wikimedia Commons This question and especially its resolution criteria part is based on RyanBeck's question about Melitopol.