When you completed a mission, you get the victory screen with below that a Score. In single player, this score doesn't really matter, it's just something for yourself: if you replay that mission, you can aim for a higher score than the previous go through.

In multiplayer, however, your score determines your rank on the player list of that game.

How is that score calculated?

Your score largely depends on the mission goals: if something is a goal you will get points for it, if not, you won't get any points. Also, your score depends on how long it takes you to complete the mission: 2 points will be subtracted from your score for every month you spend in the mission.

Table with points:

Goal type Score if it's a mission goal Score if it isn't a goal
Population city population / 10 0
Treasury city treasury / 25 city treasury / 50
Monument completion percent complete x monument value (see below) N/A
Menagerie 25 x different animal types in your palace 10 x different animal types
Trading cities 50 x trading partners 0
Vassal cities 250 x vassal cities 0
Allied cities 150 x allied cities 0
Housing level 100 per housing goal 0
Production 100 per production goal 0
Profit highest annual profit / 25 0
Hero 10 for each month in which a hero is in your city 0

Monuments are a little different: some monuments are very large, some are small. Thus you won't get the same amount of points for every monument completed. The following table lists the monuments and their points:

Monument Points
Great Temple 100
Splendid Temple 150
Temple Complex 200
Grand Temple Complex 300
Tumulus 150
Grand Tumulus 200
Large Palace 200
Underground Vault 500
Grand Canal 400
Earthen or Stone Great Wall 500
Clock Tower 100
Grand Pagoda 150

All points added up form your "Raw score", this score is multiplied by a value that depends on difficulty:

Difficulty modifier
Very Easy Easy Normal Hard Very Hard
0.5 0.8 1 1.2 1.5