From a very quick read:
SECONDARY SURVEILLANCE RADAR (SSR)
1. Operating procedures
Unless otherwise authorized by the Dirección General de
Aviación Civil, the use of SSR transponder is compulsory for
all aircraft flying:
a) Within Madrid, Barcelona and Canarias FIR/UIR at FL 200
or above, and
b) Within Madrid, Sevilla, Barcelona, Palma de Mallorca, Valencia
and its airspace delegation (see ENR 2.3) and Canarias
Control Terminal Areas at any level.
c) Within Area 1 Galicia TMA.
In case of transponder failure, pilots need a clearance from
the Air Traffic Services to fly within the airspace where its
use is compulsory.
SSR transponders will have, at least, Mode A/3 and capability
for 4096 codes, except when duly authorized, in which
case it is a must to have capability for 64 codes.
Pilots shall operate their SSR transponders and select modes
and codes in accordance with ATC instructions, and in
particular Mode C automatic altitude reporting, if available,
and shall maintain that setting until otherwise instructed.
Pilots of aircraft about to enter Madrid, Barcelona and Canarias
FIR/UIR, who have not received specific instructions
from ATC concerning the setting of the SSR transponder,
shall operate it before entry and maintain the following mode
and code settings until otherwise instructed:
a) Controlled flights: Mode A/3, code 20 (or 2000) and Mode
C automatic altitude reporting, if available.
b) Non-controlled flights: Mode A/3, code 70 (or 7000) and
Mode C automatic altitude reporting, if available.
Spanish AIP
ENR 1.6-13 refers (registration possibly needed)