(Capcom's primary storyline)
The Monument was a large tower on Sejm Island, dominating the skyline as the tallest building. It was used as Alex Wesker's headquarters until her attempted suicide in 2011, which triggered the self-destruct sequence.
It is unknown when this Monument was erected within the island, possibly it was already there even before Alex Wesker arrived. But around 2000, when Alex first arrived at the island she immediately made preparations to use it as her "throne" as well as ordering the construction of her personal laboratory. If any of the islanders ever felt uncomfortable about anything, the Monument was the place where they could meet up with Alex in order to discuss whatever they wanted to.
Around late 2008, a few villagers actually went to the Monument to find out what happened to their relatives that went missing for a year, being aware that Alex offered them a contract to work in the mines only made their suspicions grow about Alex's intentions, making them targets to eliminate. Eventually, the villagers never returned home (possibly being taken by Alex Wesker to be experimented on with the T-Phobos Virus alongside their relatives).
In 2011, the Monument had most of its infrastructure destroyed when the laboratory's self-destruct sequence was activated.