1.) Locate to your Steam Rise of Nations directory 'C:\Program Files (x86)\Steam\steamapps\common\Rise of Nations\Data\'
2.) Create a copy of 'rules.xml' file first then edit it with Notepad++
3.) Ctrl +F and find '<POP_CAP entry0'
4.) Change 'entry7="200 pop cap"/>' to 'entry7="450 pop cap"/>' **(This needs to be 100 less than the new popcap you are setting)!**
5.) Ctrl +F or find 'MAX_POP_LIMIT'
6.) Change '<MAX_POP_LIMIT value="300 pop cap"/>' to 'MAX_POP_LIMIT value="550 pop cap"/>'
7.) Ctrl +F or find '<CATEGORIES id="poplimits" title="Population:">'
8.) Change '<CATEGORY name="200"><DATA>200</DATA></CATEGORY>' to '<CATEGORY name="550"><DATA>550</DATA></CATEGORY>'.
9.) Save the file.
10.) Launch the game, upon reaching the Military Tech Level 7 you will now have 550 population cap instead of the default 200.

Q1.) How do I play online with my friends?
A1.) Your friend must have the same rules.xml configuration as yours otherwise you will recieve a sync error.

Q2.) How do I revert everything back to default?
A2.) If you didn't save a copy orginally, go to the Steam Library and right click Rise Of Nations > Properties > Local Files > Verify integrity of game cache...