First, make as many allies as possible. Advance through as many ages as you can to 
upgrade all your units. Make sure you can build Towers and Castles. Next, go directly
next to your ally's main city and surround it with towers. Build a castle every so often
on your way from your city to theirs, so they cannot counterattack. Next, build up an 
army of siege weapons (for the buildings) and cavalry (for everyone else), and position
them inside your ally's city. Finally, switch them to "Enemy", sit back, and watch the 
city crumble.