Often overlooked as a strategic point geography makes a vast difference in this game. Here's why.
Elevation - Units firing from elevated positions cause more damage.
Rocks - Light infantry recieve less damage when placed on rocks.
Rivers - Units move slower and recieve more damage walking in or crossing rivers (coupled with lower elevation rivers are deadly to be caught in).
Choke points and natural traps. Himalayas map is classic for this. Trees that completely surround a mountain so you run your raiders in one side to escape but there's only one way out. Knowing how to recognise bottlenecks and traps enables you to use buildings to try force escape into dead end spots on the map. ie there is a cul de sac to the left of your city so you place a tower on the wood on the right with horses close by. raiders arrive circling tower they see pursuers and head away from tower to other side of city. If you get behind them they will run into your trap unless they have already scouted it all.
Natural barriers - Mountains absolutely cant be crossed except by air units and forests can only be run through by Iroquois Scouts.
All of these add strategic elements to the game enabling you to build stronger more defensible cities.
Combining buildings with geographical features makes defense even better. Nearly all buildings can be placed in such a way as to hinder raiders attackers.
Enemies having to run round buildings to raid you gives you enough advance warning to get to safety/respond as enemy raiders will fire at the buildings as they run round them sounding the attack alarm.
Trees/Mountains in battle. - When seiging a city I always try to find a spot with trees or a small mountain to seige from behind. With units either side of forest/mountain the seige is almost impossible to reach and should have a gaurd or 2 anyway. If I can't find a spot like this I go for rocks to get the infantry defense bonus.
Seiging across rivers. When on the offense and seiging across a river have your cav force well wide of your infantry so they may flank and attack from the side/rear as your infantry attempt the river crossing. Spying really helps in this situation as larger armies can get killed if not microed properly in well defended attack spots. I will often try a dummy attack elsewhere to lessen defenses on hard to get cities.
Nice strategys D