Conqueror! Game

"Conqueror is a turn-based, multiplayer strategy game that takes place on a map of medieval Europe. Easy to learn, but far deeper than Risk. Play on the web against as many as 15 human or AI opponents, for timed games as short as 30 minutes or as long as 4 hours. An ingenious system of simultaneous turns means that you don't have to wait for other players to make their moves!" - From the Conqueror! site.

What's great about this strategy game is it's entirely free, doesn't need an account to play. Basically you can play instantly, and it offers both single-player and multiplayer. Players are also able to join midway through games (if the game is not full) and choose to play as one of the AI players.

When entering a game, choose your position carefully. This is medieval Europe, so starting your own Roman Empire may be tricky. Spain's a solid choice for players starting off as it's better protected, and only a few players can attack at a time. Russia is a great choice as the player's empire will be able to spread to Scandinavia, downwards to Slavia and the middle east, and across central Europe. Usually the winning position for an experienced player is Poland area, provided the player is good enough to face multiple enemies at once. I'd also recommend reading the guide to work out how resources, population and spending work.