Why is it "De" vackra tornen? I found an article and (though I must admit I have not read the article extremely thoroughly) thought it only depended on the last part of the noun - in this case I expected it to be "Den": http://www.thelocal.se/blogs/theswedishteacher/2010/11/08/den-or-det/
See the discussion at: https://www.duolingo.com/comment/5688144
Indefinite: ett vackert torn (a beautiful tower) – vackra torn (beautiful towers)
Definite: det vackra tornet (the beautiful tower) – de vackra tornen (the beautiful towers)
Indefinite: en vacker bild (a beautiful picture) – vackra bilder (beautiful pictures)
Definite: den vackra bilden (the beautiful picture) – de vackra bilderna (the beautiful pictures)
Same reason why the g in the days of the week (i.e. tisdaG) is usually dropped, why det often lacks a "T" sound and why jag/ja can be slurred as one sound unless you listen carefully... According to my dad it's similar in equivalency to how in UK accents often have th-fronting (i.e. saying "bruvva" instead of "brother").