I only play against expert ai and I have been using the portugese sonic boom strategy to age up to tier 3 faster than the enemy. I can hit fortress age by 7:30 and start pumping out settlers from 3 Town Centers and out boom the expert ai, but lately. it seems to have learned my tricks and is getting much tougher. so I am starting to lose more than I win. When I know I have lost, but not beaten yet. I start preparing for my last stand.
I have 2 factories producing cannons and I got 7 cannons when my last stand began, but when the ai killed my town, he left my factories and houses alone. so my cannons kept building and I had 13 at one time.
Eventually, the ai returned and destroyed my factories, so thats when you'd say my last stand really began.
I found out a really good formation that is nearly unbeatable.
Take your cannons and find a open area and clump them all together tightly, Don't use less than 10 cannons, but the more the better.
Take a lot of Musketeers (50 or more)and set them to defend stance and select them 10-20 at a time and right click on the center of your cannons. They will tightly surround your cannons. Do this again with another 20 musketeers until you used them all up. Now you just wait for something to try and attack.
This formation is no good at attacking, but it can hold an area for a long time without hardly taking any losses, and it does even better if you have priests healing it.
It will even repel a direct cavalry charge without any losses. The one weakness is against massed field guns. The ai loves to make like 20 field guns at a time and drive them right next to my last stand spot where my heavy cannons will wipe out half of them before they unpack, but once they do unpack and fire, I lose a lot of musketeers and 1 or 2 cannons.