This system does not allow me to edit my post as a junior member, so I had to reply to my own post to add to it.

The other thing about building new units in the opening of the game, since I'm waiting for better buildings to build the better troop types, I concentrate on building fleets -- which I can afford, because of the money generated by my diplomat and because they have a low upkeep cost. Early naval power is invaluable; and I upgrade their weapons and armor as soon as possible, of course. I also focus on building spies so that I can have one in each city and a few to move around with my armies and the map. Fleets and spies and a few additional diplomats take priority to military units at this early point in the game because of their low upkeep; by the time I have a good naval force and spy network, I'll be ready to start building some of the more advanced military units.

Another thing I often do, any time during the game, is build a fort and with a spy in it rather than building a watchtower. It costs a little in upkeep, but is far more flexible. I garrison the fort with a single, fast moving, low upkeep cavalry unit so that if he is besieged, he can be used as "reinforcements" (which actually make it to the battle in time!) after his rescuers attack the besieging army...This works great for blocking enemy movement through passes, bridges, etc., including blocking rebel armies from wandering into your territories.