Some of the strategy games deal in the realm of science fiction, others in fantasy, some in damsels in distress, and some in current world dilemmas. They all have the same underlying premise. Your prowess and skill in undertaking the tasks, often called quests, determine how well game-play progresses.
Some of the sites allow cooperative play, where players either play in opposition or cooperate in undertaking a mission of quest. Players from all parts of the world can play together, never meeting each other, but developing strong friendships anyway.
There are many sites that have paid versions of their ventures, even some which require monthly subscriptions. This often results in huge investments of money to keep up with the challenges posed by other players. However, there are sites that offer free play, never requiring a penny of money, just that you register and provide an email. This is mainly to prove that you are a real person.
One such game that is free to play, offers a great challenge, and is based on skill and strategy, is ROM Island. The action takes place in the future. An asteroid, having a gold-like substance, was scattered all over earth, with the center landing on a remote island. It produces a magnetic field that can produce limitless energy. Whoever controls the island controls the substance, and, subsequently, the world.
The object of the game is to conquer the island and hold it against all competitors. Your ability to defeat others who are after it determines you success. The world has changed, with continental blocks and militant groups. You compete against them for control of the island.