Exegesis:

pros thuran ‘at the door.’

exō epi tou amphodou ‘outside in the street’: so most commentators and translations.

amphodon (only here in the N.T.) ‘a city quarter’ (cf. Moulton & Milligan): by extension of meaning ‘the street (or, road) around (the quarter).’

kai luousin auton ‘and they loose it,’ i.e. ‘and they untie it.’

Translation:

Found a colt tied at the door out in the open street may require a division into two or possibly three paratactically combined sentences, because of the change in subject, e.g. ‘they found the young donkey; it was tied by the door; it was standing out in the street.’ Where the passive it was tied must be changed to active one may say ‘men had tied it.’

Some translations have taken amphodon in the sense of ‘cross-roads,’ e.g. ‘where roads come together’ or ‘where roads separate,’ but this is not the meaning of the Greek. This term has reference to the larger streets which encircled smaller quarters of a town, through which led only small passageways, often completely covered over.

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .