The best way to defend a continent is to put ALL the arimes you wish to dedicate to the defense of that continent on the border, ON THE BORDERS - coz that is where you will get attacked. Ignore all these people that have 5 backed up by 3 backed up by 2. Its the bonus you want to defend.
For example if you wanted to defend north america on the classic map divide your forces between Central America, Alaska and Greenland. You may want to distribute unequally according to threat at a particular border.
If you own places like kamchatka and iceland then fort to there so as not to trap your forces.
Finally - remember that a truce.alliance/pact with a next door neighbour is very effective and worth thinking about.
Rocketry