Cardiff will allow long-serving Kevin McNaughton to leave this summer along with Danny Gabbidon and Nicky Maynard.
Highly-rated Atletico Madrid midfielder Saul Niguez has signed a new five-year contract.
New Watford midfielder Alexander Merkel says he turned down offers from Bundesliga clubs to join the Sky Bet Championship side.
The 21-year-old last week joined the Hornets on loan from Udinese for the rest of the season, dropping down a tier compared to the top flight of Italian football and saying it was "easier" for him to move to England than to Germany.
"I had offers from clubs in the Bundesliga and 2. Bundesliga," Kazakhstan-born Merkel, who has represented Germany at youth levels, told Sport Bild magazine.
"But in view of the co-operation between Udinese and Watford, it was easier to take this step."
Watford have signed a number of players on loan in recent years from sister clubs Udinese and Spanish side Granada, all of whom are owned by Giampaolo Pozzo.
Former AC Milan and Genoa player Merkel will be looking to get his career back on track in England after losing his way in recent seasons.
After making his debut with Milan in 2010, a combination of injuries and bad form subsequently saw him struggle to hold down a first-team place at either Genoa or Udinese, leading to his switch to Watford