As stated in the description there are four modules. One of
them I
consider not super useful (the bonus cards), two of which
are kind of
interesting (the vizier and the exchange cards) but one of
them really
adds to the game and is a partial patch for one of its
weaknesses.
Basically, the builder's huts allow you to multiply the
value of the
pieces you have purcahsed. More than that, however, they
also allow
you to expand your wall or get you out of a building jam. They
therefore enrich the basic game and if you are a fan of Alhambra make
it almost worth the purchase price. (I'd give this expansion
4 stars
if it were just the builder's huts and priced accordingly.)