Just in general, a military victory is by far the easiest to obtain.

Each victory should be equally obtainable, but if you go for any other victory someone who's going military will swing through and wipe you.

Granted some of the other victories would break in MP if you got close to them due to player actions (players get the warning that you're close to Diplomatic/Economic victory, so they all close their borders and declare war on you)

It is less expensive to defend than it is to attack.  Build defensive structures, full stack of archers, and build the wards from the pearls list to turtle.  Build the xp per turn buildings as well.

I might argue that the game should not encourage players who "turtle in" and pursue economic/scientific/... victories. Instead, I believe that they should need to make investments toward military units and military tech. When exactly those investments need to be done is the whole point.

Disagree. Diplomatic victory is the easiest to achieve. Start a war with an enemy you can win, take a few cities. Trade for a few more of their cities to call a truce. Crippling them for the rest of the game. Offer peace and then an alliance. Use them as a buffer between other enemies to repeat this process across the map. Okay maybe it's more work. :p