City limits

I am fairly new to this game. I'm having trouble expanding. My city appears to be size '3' according to the map. I have two oasis and now I am unable to get any more or occupy fortified points, because this exceeds city limits. What are the restrictions of city limits? How do I expand? Do I have to battle neighbouring cities?

Ok well here's the list of how many sub cities, oasis' and forts you can have at each lvl:

Lvl Branch Oasises Forts
1       0         1         0
2        0         1         0     
3        0          2         0
4        1          2         1
5        1          3         1
6        1          3         1
7       2          4         2
8       2          4         2                                   
9        2          5         2
10     3          5         3
11      3          6         3
12      3          6         3
13     4          7         4
14      4          7         4
15      4          8         4
16     5          8         5
17      5          9        5
18     5         9        5
19   6          10       6
20   6          10       6      

I know it's a bit wavey, but it shouldn't be too hard to understand still lol.

Also, another way to work out how many oasis' you can have, half your level and round up. Eg; if your lvl 3, half of 3 is 1.5, round up and you have two, so you can have two oasis' at lvl 3

And for forts and sub cities, you can have an extra one for every major title upgrade. Eg; 3rd Baron, 3rd Marquis, 3rd Earl etc.

Thanks, next obvious question how do you raise you level? Are some ways more effective at raising your level early on?...

you raise your level by earning prestige_

ways to gain prestige include,
-training troops
-upgrading buildings
-Hunting
-killing other peoples troops.

Prestige: General's prestige, together with treasures, is prerequisites for title upgrades. Your prestige is gained from completing task, upgrading buildings and techs, making soldiers, and attacking other people. You may lose prestige when you lose an attack. Prestige is used to upgrade your general level.

Courtesy of the help section