construyeron/construidos
En castilla la mancha puedes encontrar muchos castillos construyeron/construidos por los cristanos y musulmanes.
In the sentence above which word would be better, construyeron or construidos? My guess is construidos.
4 Answers
I think you could either say:
En castilla la mancha puedes encontrar muchos castillos construidos por los cristanos y musulmanes.
o
En castilla la mancha puedes encontrar muchos castillos. Estas castillas construyeron por los cristanos y musulmanes.
The sentence structures would have to be different.
Please wait for confirmation from a more advanced Spanish practitioner. 
For example you would normally use construido in a sentence like:
Esta estatua fue construido en 1939 por José Hernandez para conmemorar la liberación de la ciudad de San Sebastian por capitán Sebastian Garcia Cortes
= This statue was constructed in 1939 by Joseph Hernandez in order to commemorate the liberation of the town of San Sebastian by Captain Sebastian Garcia Cortes
This information is entirely fictitious to the best of my knowledge and the details were plucked from my head to illustrate a point: how to use the word construido (past part of construir)
I hope this helps 
I would choose construidos but don't take it as gospel.
My thinking is this "Los castillos fueron construidos por los cristianos."
Construido
I would use construyeron por los ...were built by the...preterite verb. Construidos here (i think) be an adjective?
