Unknown Coin

Where was the field? The top photo shows a man with a halo, possibly a saint?

Indeed, it is a saint. The beginning of the saint's name is legible: S PE-- He seems to be holding a book in his left hand, but in his right I think we can see the key of Saint Peter (S PETRVS).
The coins that most appropriately feature St Peter, are, of course, the Papal coinage, which were struck for circulation in the Papal States. Above each top corner of the shield on the reverse of this coin you can see a small circle. These are the bases of the two crossed keys, the crest of the Papacy.
Starting in the early 15th century, Papal coins started bearing this design, with the Pope's family arms topped by the crossed keys and the Papal tiara. As for the arms, even in this picture, you can see at the very bottom of the shield the trunk of a tree. This is the oak tree of the arms of the della Rovere Dukes of Urbino. The House of della Rovere produced two Popes, and if you can give us a better picture of this side of the coin, we may be able to read in the legend either SIXTVS, for Pope Sixtus IV (who created the Sistine Chapel, which is named for him), or IVLIVS, for Pope Julius II (who ordered Michelangelo to paint the Chapel's ceiling).
I don't have a good reference for Papal coins, but I think this one is likely a quattrino or picciolo, made of billon.

http://www.rhinocoins.com/ITALY/SCH...SISTOIV.HTML
http://www.rhinocoins.com/ITALY/SCH.../GIULII.HTML