Correct Multiplayer Game Development?

I am going to be making a turn based strategy game in the vein of X-Com. Going into making my first multiplayer game and reading up on the documentation, following tutorials and looking at other resources, I still haven't found a few things out. Primarily of which: Would I be better off Developing most of the game, then implementing the networking OR Implement the network along with the mechanics of the game?

Also, if you have any resources at all that could be of help, that would be fantastic!