Age of Empires III Walkthrough

2.2.8  Bringing Down the Mountain

Have all your miners work the northernmost bridge.  Send your hero, two 
Culverin artillery and soldiers to the middle bridge, and place them 
overlooking the passage.  The first cannon will go through here.  Bombard it.

In the meantime, create Musketeers and Skirmishers and send them to the
Culverin.  Use a card to get more Culverin.  More miners should appear; have
all of them work on the top bridge, too.  Your northern base may be raided by
Cossacks and other units.  Just have your Settlers garrison in the Town Center
and they should be able to ward off the attackers.  You may lose the market but
that is a small loss (100 wood and the build time).  Get a Fort Wagon with a 

Have the two new Culverin you sent for occupy the ridge immediately under the 
middle rock bridge.  The next wave comprises of two cannons.  By now your
miners should have destroyed the upper bridge.  Have them work on the middle
bridge.  Your Culverin will demolish the enemy's cannons.

Miners are hardy units with powerful explosive attacks (they lob bundled sticks
of dynamite).  Keep one around to wipe out units and have the rest work on the
bridges.  You can have two Culverin guard the northern most passage (this one 
does not have a rock bridge going over it) if you are obsessive about 
preventing all of the cannons from getting through.  Otherwise, have them 
reinforce the other cannons.  By now your Fort Wagon should be here; build the 
Fort on the ridge between the middle and the lowest bridges.

Build soldiers, destroy as many cannons as you can, and keep your miners 
working.  The Fort will be helpful.  

You win when the final bridge is destroyed.