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Alhambra is a game ahead of its time, mostly in the inclusion of a large number of small mini-expansions which can be mixed and matched freely with the base game, depending on how far you'd like to change the base game.
Components and Gameplay
Like most of the other mini-expansions, the components are fairly minimal, in this case a set of 6 cards, each representing an opportunity to change money between a pair of the game currencies. There is only a single card for each pairs of currencies.
These cards are shuffled into the money deck, roughly in the middle-game and are then turned as normal money cards. They remain face-up alongside to be taken just as with money cards, although you may only take a single card.
When paying for any tile, players can discard an ancient travelex card from their hand in order to pay with any combination of the two currencies shown and buy a tile that is up for sale in either of the two different currencies. For example, using the blueyellow exchange card allows a tile being sold on either the blue or yellow market to be completely paid for with blue cards, or yellow money cards, or any combination. The bonus of continuing your turn for exact payment also works normally.
This is one of several expansions that aims to shift the balance of how important (and how difficult) it is to buy things with exact change. Unlike the change coins, there is a significant cost to gaining one of these cards (a whole turn spent), but they can be immensely useful in overcoming issues with having run out of the precise currency needed for a building.
I find this expansion most interesting because it adds a little more dynamic range to the value judgement in the choice of whether (and when) to pick up money. The change cards certainly aren't overpowered, since they can only be used once. At certain points in the game, having an easier time of getting bonus actions can be really important (when there are a couple of amazing tiles, for example)
Possibly not a great choice for the first few games, but I think this mini-expansion is a nice addition when you're chasing a little more variety.
I've created a Geeklist of all the Alhambra expansions including community polls on which are the most essential mini-expansions.