A long Investment.
There is always a race that you haven't tried, a map you haven't built on, a scenario condition you haven't challenged and a warrior you haven't created before. The game just goes on...
Some fabulous ideas of border expansion and a great portray of Politics involvement in it. Say for example, that another nations sub has been detected within your Sea border or another nations plane is suddenly flying within your airspace, heading for your capitol...
What do you do? Open up the political bar were you can Negotiate with the other Nations leader, you can bribe, insult, pleas, trade, negotiate, warn, declare war or make treaties/ allies with nations. It just keeps on going...
Probably the most rewarding part of the game is Nations coming to you in scars offering you money or land/ cities after you started to raid their borders and capture city after city...
Its one of those games were you really have to plan for the future and ill give you a tip, read the Civilization encyclopedia and see stats and technology that can lead to such and such.
Preparing a fleet to lead your armies across the vast array of seas and planning were and when your going to land is just a massive choice that can impact the whole world in the game, literally. Just because you capture one border city doesn't that they are okay, they could have stored many things in that city and might have been their only harbor city, or have many monuments in it...
Future planning in the game is vital and overall is a game that is quite hard for youngsters and yet is also for adults, but that is what makes it fun.