Castelldefels is located in the province of Barcelona, at 41 20' North Latitude and at 2 40' East Latitude. It is the last city going down south the Baix LLobregat, between the Delta of this river and el Massis de Garraf. It limits From East to West with the coast line, about 5 Km. of the Mediterranean Sea, and, from North to West with the first reliefs of Garraf. The administrative limits coincide on the North and the East with the city of Gava, and on the West with Sitges. Castelldefels is part of the Catalonian Community.

Castelldefels, a city on the outskirts of Barcelona, communicates with this big city by roads, C-245, C-31 (Autovia de Castelldefels) and C-32 (Free Highway from Barcelona to Castelldefels). Two train stations are avaliable : one in the city centre (Castelldefels Pueblo), and the other on the beach (Castelldefels Playa o Apeadero de Castelldefels). Both are from the same train line Barcelona-Villanova i la Geltrú. There is also a bus line to Barcelona City, which toghether with the International Airport of El Prat de Llobregat, at 7 Kms by the C-31, complete the city communications.

Castelldefels has a tipical mediterranean climate, this means rain in spring and autum with spectacular storms, which can pour 50 litres per m in less than an hour. Dry summers and winters when you can fel the influence of anticyclons. The termic regulator effect of the sea provides mild winters and hot humid summers. The average temperature around the year is about 16C, and about 300 sunny days a year. The rain rate is 600/700 L. per m/2.

The extension of the area of Castelldefels is 1.274 Hectares, half of this area is urbanized, and another 427 are dedicated to urban residential land, which represents 34% of the whole.

The present population of Castelldefels is more than 40.000 inhabitants.