Casitas 1 and 3 are closest to the convention center and El Centro. Parts of Casitas 2, 3 and 4 are closest to the spa, salon, fitness center, guest laundry and quiet pool.
The casita quiet pool is WDW's only full size lap pool, great for those guests that like to get their swimming excercise in while on vacation. CSR is the only moderate resort to offer any of these amenities.
Casita 4 is nearest to bus stop #2, which is the first stop for all theme park buses. Casita 5 is the furthest out from the El Centro but closest to the Dig Site.
The preferred rooms (buildings 1, 2, and 3) in the casita section do offer a covered exterior walkways to and from El Centro if visiting in the rainy season. Casita 1 is even attached to the convention center for access to El Centro during inclement weather. You can also get to bus stop one using covered walkways.
Hammocks were recently added to this side of Lago Dorado in front of casita 5 so you may enjoy the beauty of the lake and its gorgeous sunsets. It's a little batch of heaven.