There are two expansion packs for Civ 4: Warlords and Beyond the Sword. They cannot be played simultaneously, and must be played separately. I have played them both extensively, and I prefer Beyond the Sword. It is a vast improvement over Warlords.
I love Flood Plains, because they provide an extra unit of food, allowing for greater population growth. Of course, you then run the risk of having an unhealthy population (where the number of unhealthy citizens exceeds the number of healthy citizens). To minimize the risk of unhealthiness, do the following:
(1) Build Aqueducts. Cost: 100 hammers. Technology Requirements: Mathematics & Masonry. Benefits: +2 Healthy Citizens.
(2) Build Granaries. Cost: 60 hammers. Technology Requirement: Pottery. Benefits: +1 Healthy Citizen (with Corn, Wheat or Rice), +50% food storage after growth.
(3) Build Grocers. Cost: 150 hammers. Technology Requirements: Guilds & Currency. Benefits: +1 Healthy Citizen (with Banana, Spices, Sugar, Wine), +25% gold/turn, can change <= 2 citizens into merchants, required to build Supermarket.
(4) Build Harbors (coastal cities only). Cost: 80 hammers. Technology Requirement: Compass. Benefits: +1 Healthy Citizen (with clam, crab, fish), +50% trade route yield.
(5) Build Hospitals. Cost: 200 hammers. Technology Requirement: Medicine. Benefits: +3 Healthy Citizen (with clam, crab, fish), Heal units extra 10% damage/turn, required to build Red Cross.
(6) Build Supermarkets. Cost: 150 hammers. Technology Requirements: Refrigeration (and Grocer). Benefits: +1 Healthy Citizen (with cow, deer, pig, sheep), +1 Food unit/turn.
(7) Build Public Transportation. Cost: 150 hammers. Technology Requirement: Combustion. Benefits: +1 Healthy Citizen, +1 add'l Healthy Citizen (with oil), +1 add'l Healthy Citizen (with Environmentalism).
(8) If all else fails, you can "Turn on Avoid Growth" for any city that you choose. Click on the city's banner to bring up its menu display. On the right side, you will see a column with the heading, "Draft." In the bottom right corner of this column is a "Turn on Avoid Growth" button, which you can select if you want to prevent further growth in a city that has too many flood plain squares, or is otherwise growing so fast that you are currently unable to control unhealthiness. Hopefully, you will later be able to allow full growth, once your technological development allows you to build all of the city improvements necessary to manage large populations.
