There is one aspect more of the question, namely whether you could read the old texts as they actually were written. And it is not only a question of changes in the alphabets. I can read Latin (though not without some lookups), but inscriptions on graves etc. are so stock full of abbreviations that reading the stuff sometimes feels more like riddle solving than as free reading. Any saga in Old Norse you are likely to see will also be changed into a standard orthography - and in the Old French writings you will often find that dialectal traits have been edited away. For the scholar this is irritating (bordering on sacrilege), but for struggling learners it is probably a good thing.
As it is I can spell my way through almost all the Romance languages back to Latin, including the oldest versions from around 800 (give or take 100 years), and French is probably the one that has changed the most. But even the Strasbourg oaths from 842 are legible, although they like like the missing link between Vulgar Latin and 'real' French. The one language that may be problematic is Romanian, where I long ago found and studied a
text from somewhere around 1521. I have forgotten the details, but remember that the thing is was almost illegible to me - and probably most Romanians too.
EDIT: now I have reread it, and it has not become easier in the meantime.The old Germanic languages except Old Norse are slightly harder, but not so much that they have become totally opaque. This also covers my experiences with Gothic. But after several bouts of Beowulf wrestling I can spell my way through it - well enough to see how dubious some of the published translations are. 'The Perl' is just about the limit where I can read freely - skipping some unknown words along the way.
Old versions of the Slavic languages are beyond me, and as for older versions of Greek I can understand some words here and there, but not read them. Some day I may find time to learn something about these phases of the Greek languages, but so far I'm just satisfied that I can read non-fiction non-newspaper Dhimotiki.
"oubliette - very small one man cell"
: It was even worse. I recently watched a documentary about Warwick castle, and there the cell was at the bottom of a shaft leading up to the toilet seats, and from other sources I vaguely remember that food for the unlucky inmates wasn't provided. In at least one other case there was not even a stair down to the bottom of the pit so if you were lucky you would break your neck at the arrival.
