Roll Through the Ages has a few expansions and variants, and Iron Age is one of the variants. It's more a variant than an expansion because it takes out some parts of the original and replaces them with new elements. It's not just adding minor rules or small modules.
The player board is different. You don't have 5 types of goods. You just have goods (fourth row, grey). Grain (5th row, green) remains the same. If you expand your empire by settling more provinces, you will need more grain to feed your population every turn. The first row is gold. You can convert goods to gold to be used on a future turn. This is akin to investing and earning interest, because gold is worth more than the original goods it is converted from. You spend both gold and goods to discover new technologies. The 2nd row is ships. Once you discover the ship-building tech, you can build ships. They are worth points, and can also be used for war. The 3rd row is armies. You spend food and population to build armies, which are of course used for war.
Warfare is a new concept. Some die rolls let you decide to initiate a war of conquest, which lets you score points depending on how large your army is. Some die rolls force you to enter a war. You win or lose points depending on how well you fare. You don't directly attack a fellow player, but your military strength difference does come into play because of the other new mechanism - the tribute. One of the die rolls let you demand tribute, which is basically scoring points depending on how much stronger you are compared to every other player. The others don't lose points, but they can deny you points by paying you one good.
This is the player sheet used in Iron Age. In the base game, the more cities you build, the more dice you roll. There are no cities in Iron Age. Instead, you get ports and provinces. Both allow you to increase your dice. Ports increase your rate of collecting goods too, and do not consume food. Provinces increase your military strength and award tribute points. The turn sequence is very similar to the base game. You get to roll up to three times, just that dice showing skulls (disasters) are locked and cannot be rerolled. After rolling dice, you collect goods, workers and food, and then spend them to build ports, settle provinces, build ships, build armies and construct monuments. You need food to feed your people. You may spend goods, gold and innovation points to discover a new technology. The possible new phases are warfare and tribute. They do not occur every turn.
The game can end in three different ways - when a player discovers a specific number of techs, when a player scores a specific number of tribute points, or when all monuments are completed.
This yellow fate die is new in the Iron Age. It can affect your harvest. It may allow you to initiate a conquest or demand tribute. In this photo, the smoking bones icon means a good omen - you can set this die to any face you want. On the left side of the die, the helmet icon means you have the option to initiate a conquest. On the right side, the sun means a drought, and the food production on every die is reduced by one. Other than the fate die, the basic dice in the game have also changed.
I did a four-player game. Roll Through the Ages is a filler, and can be completed in about half an hour to 45 minutes. It is quite straight-forward, but it has some strategic depth. How it uses a dice mechanism to tell the story of a growing civilisation is interesting. In the Iron Age, a slightly different story is being told. You deal less with specific goods, and you have to manage the arms race more. Iron Age felt more like a full-fledged boardgame than a filler-type dice game. You have more decisions to make than the original game. Our game ran longer than an hour. I'm not sure whether it was more because we were unfamiliar with it, or the game is indeed more complex than the original and takes longer to play.
If you like Roll Through the Ages, you should give Iron Age a try. The basics are familiar, but the overall feel is a little different. If you are new to the series, I would recommend starting with the original game, which is more straight-forward and plays more smoothly, but still has decent strategic depth. Iron Age is more for players who already know the original, who already like it, and want to inject some new elements.