The following description is from dminoz's "Daily Imperialist". (Authorship and copyright not certain, and his e-mail seems to be dead; if he or anyone else claiming authorship thinks this should not be here, let us know.) :
- "The Builder is the unit responsible for all land tile upgrades. It's the Builder who opens up the natural wealth of your empire, enabling you to exploit it. Of course, you should have more than one.
- "On the Normal or Easier levels, you should get a second Builder right away. On Hard and Nigh On Impossible, priorities are a little different; you build a Spy first, but a second Builder follows immediately afterwards (if you can afford it).
- "Each section of the road built by your Engineer connects the tile it's on and all surrounding tiles. That means that there is much more work for a Builder than for an Engineer. In fact, after connecting roads to selected spots, your Engineer can probably take a break while two Builders labor away on the freshly connected tiles.
- "In spite of all this, don't go overboard with new Builders. Your economy won't ever be able to support more than three Engineers and four Builders; they'll be using enormous amounts of lumber and cast iron. In any case, three Engineers and four Builders are more than enough to quickly develop your empire.
- "The only time when you might consider adding even more Builders is when a Minor Nation joins your empire. You'll be racing to develop food-producing tiles in the shortest possible time, as you'll inherit the Minor Nations armed forces and thus be responsible for their upkeep."
There could be debate about the second quoted paragraph. If your builders get too far ahead of your road and port access, you may run out of lumber and/or cast iron and leave them with too little to do, thus wasting the $500 and paper you spent on the second one. A priority, if you are on a random map, is to find the nearest iron ore, creating a second or even a third explorer to do it fast. Get road access to your first timber and iron ore and develop them, because there's no guarantee that you can buy all that you need; then check your per-turn income of lumber and iron ore, to see whether a second builder is a good idea, maybe to work on food tiles to expand your population, then on other resources.