I'm not going to walk through the economic aspects
The First thing I do is use my Fort card for my first shipment and build it just s bit NorthEast of the starting Town Center. Then I make a wall that touches the north side of the Fort and goes west to the edge of the map. Then I leave a gap and make a 2nd wall going to the coastline from the fort. I leave a hole here so the ai can still enter my base at a controlled chokepoint. This chokepoint is about as wide as the Fort. I use my 2nd shipment on the 16 musketeer card and defend the chokepoint. Just stick them right in the hole to block enemy units. Those enemy cavs will run straight into their bayonets. This is the point of the game where I am weakest. If they do a fast rush and kill those musketeers off before my economy is strong enough to replace their losses, I can easily be overwhelmed. Next, After I have about 30 musketeers holding the chokepoint. I make field cannons. First right behind the infantry, and as I build more, I place them along the wall to soften up incoming enemies. Once I have the field cannons in place, I am fairly safe.
I forgot to mention, I use the card that lets you upgrade your fortifications for no cost. That means free wall, tower and fort upgrades. And I use the card which increases the tower limit. I place the towers along the wall to kill the straglers who will throw torches at the wall when I'm not paying attention.
After The base is set up, I focus on getting the upgrades. You'll usually run out of food and gold quickly once you turtle up, so I get mills and plantations up and upgrade them. Its important that you upgrade your musketeers quickly. And have some priests behind them to heal after every battle.
Things can get tricky though. like the game I just played. Yellow attacked from the North with many many cannons and broke down my wall and tried to raid my villagers. I took my musketeer army in to kill them all and repaired the wall quickly. Most of the damage was easily fixed since I had a strong economy.
Its important to have a 50/50 mix of the field cannons and the culverins behind your walls. and you need to micromanage to make the culverins shoot at the enemy cannons. Culverins are very powerful against artillery, taking out enemy cannons in one hit most of the time. You need a priest to heal your cannons too, because they are too costly to replace all the time.
After some time passes, you have fully upgraded your musketeers with every available upgrade including HC cards. Your musketeers have some very awesome stats. Mine were at 420 HP, 60 damage range shot. A group of 40 of these guys can kill an enemy Fort in about 15 seconds, honest. I build an array of barracks just outside of my walls and assign all of them to a hotkey and queue up to the pop cap on musketeers and flood attack the AI base. Aim for the TC and it'll go down quickly. you need to micromanage to take down enemy cannons quickly, they are the biggest threat to the infantry flood. Set a couple of guys to melee mode and attack the cannon with them. Having so much HP, the cannons aren't one hit kills anymore.
Thats all I do, is make cannons and musketeers, not mortars, they won't shoot at units. I have a special deck for this called my Turtle Deck