Infiltration

This strategy consists in infiltrating the core of the enemy.
There you can do whatever you want to do, but there are two things you can perform well:
Build a full OLU city, or a OLU city with CS.
As you are in the same island of the reds, you can check thier harbors every time with small groups of slingers for example.
Probably you'll be attacked by everybody, so stacking units in the city is essencial. Birs or DLU, if you have a wall lv 15 or higher.
For these reasons many alliances like to fill their islands, even if the city has low points or it's just an anchor, ready to be colonized.