|Rare resource||Oil patch||Fish / Whale||Ruins|
Be sure to seize and maintain control of the center of the map as it is the only place that offers rare resources. To do this, be sure to build redoubts or a navy to assert your command over the area. In fact, if you have constructed several Military Buildings already, it should be easy to quickly attack and fend off invasions.
However, if you do this, enemy nations will also display interest (and perhaps concern) over your expanded territory and control over the rare resources. To counter this, be sure to have a strong air force and deter the opposing force from eliminating you.
It is important to consider that you must also obtain other resources, such as metal and oil, if you want to sufficiently maintain a healthy economy and be able to compete with other nations. Furthermore, having a surplus of such goods may be able to allow you to have allies, if democracy (and negotiation) is enabled, or even produce large quantities of military troops to intimidate enemies.
Since moving your military can be quite a hassle in some situations, try to devise a distraction or command your air force to bombard opponents. Furthermore, conducting a siege can make it easier for you to conquer cities.
If your cities/units are being attacked nearby the water, it is possible to construct a small navy and fend off the invaders. Generally, combined with this tactic, you should be able to attack the enemy from at least two or more sides. Moreover, constructing Forts/Redoubts can keep an eye on future invasions.