I find that I will not leave my harbors with anything less than 3 to 4 transports fully loaded with my best ATTACK unit. If I feel that the counter attack will be feirce I may bring 2 or 3 of my best defensive unit. But no more than that. anywhere between 2 to 5 frigates or destroyers for escorts, as the computer never attacks my incoming invasions. I choose a site in which my units land on a Hill or Mountain tile so that I can reep the benefits of the defensive bonus during the turn lost to disembark. I hit the city I landed a tile away from with every thing I have and send my transports home (like Cortez's men, my troops have no choice but to fight to win). Once I take the city I then attempt to send my Stack of Death (SoD) out to my enemies most prized strategic resources to cut off any attempts to build a counter attack army. Then I will come back with my transports fully loaded again with my best attack unit and lay waste to my enemy's empire.