The appartments are located in a large compound consisting of several blocks, with access from both Merced and Huerfanos. They are conveniently located in the heart of the city, at walking distance of all main sights, with the Santa Lucia hill located next to the compound (do go up there for a nice view of the city in several direction).
The compound has its own swimming pool on the roof of one of the blocks. It's pretty basic and the water is rather cold, but it's large enough and thus sufficient for an occasional swim. Don't expect anything too fancy though.
The appartments all have the same size and more or less the same type of appliences and furniture. WiFi with good speed is included.
There is a supermarket in the premises on Merced Street, which is even open on Sundays. Nearby there are several places where you can have breakfast in case you don't want to cater for it yourself.
On the downside: if you have an apartment facing Merced Street, it can be pretty noisy and the windows are far from sound-proof.
All in all a good deal with good value for money. Note that there are several providers who rent appartments in the same compound, all offering more or mess the same service but at different rates. So better compare well in order to find the best deal. I stayed in the compound on two different occasions and with different providers (see my review of Departamentos Amoblados Costa Bellas Artes). However, Apart Boutique Bellas Artes was the better deal: slightly more expensive but located on a much quieter side of the compound, not facing the busy Merced Street and higher in the block. The services was also better, was very friendly people ready to assist and give practical information and advice.
You can book through a booking engine, but if you contact them directly, you may be able to negotiate a discount.