Say a time traveler, dressed in the common style but not having done any other studying on history, waltzed into a market in the later end of the era of castles and asked "so, what's been happening around here?" to some random passing stranger, how well can we expect this conversation to go?
Hi, not discouraging anyone from taking a stab at your specific scenario, but you may find the following posts interesting:
How far could I go back in time and still be able to understand period English and communicate effectively? - /u/traininthedistance kicks off the subject with a brief introduction to the timeframes of various Englishes. Note the reference to The Canterbury Tales vs Sir Gawain and the Green Knight, which leads us to...
How is it that I can mostly read "The Canterbury Tales" in the original, but "Sir Gawain and the Green Knight," written at roughly the same time and in the same country, is mostly unintelligible? - recent post (so you can join in the thread with follow-up questions if you like) - featuring /u/sunagainstgold and several other users in a long conversation on English in this period
If you took an English person from 1314 and an English person from 1114 and got them to talk to each other would they think each other's speech patterns were odd? If "yes" would they still be able to easily understand each other? - featuring /u/popisfizzy and others
A couple of more general posts:
FAQ entry What is the furthest year I can go back in time to England and be able to communicate with people? - featuring /u/texpeare and others; features a few sound samples
What's the furthest I could go back in time in, say, London and still generally understand the English being spoken? - featuring /u/jimthewanderer
For all of the archived posts, if you have follow-up questions, just ask here & tag the relevant user's username to notify them