Yes. It was a fairly orderly decay as crumbling of civilisations go. Compare the decline and fall of the Roman Empire in its later years; gradually the ability to hold territory decreased until only the core of the empire remained, whilst the other regions deprived of their connection with the central empire crumbled into decay and stagnation. Much knowledge was lost by comparison.
The Old Empire is more technologically advanced than you are right now, certainly. It won't make it much easier for you to research techs in terms of the 'another faction has it' bonus, and it won't count towards the 'five players to public domain' bonus. But the Old Empire and some of the richer neutral territories will have access to tech you have not yet researched. Conquering those territories will give you bonuses to researching said techs.
The Old Empire is absurdly powerful by comparison to any of the current player empires but it is so tied up in bureaucracy, corruption and isolationism that it is unlikely it will ever bother to invade other countries unless it is provoked into war. I have been asked about making trade treaties with the Old Empire, and my answer is that the OE will not even recognise you as a valid nation until you have at the very least united a former country (such as Garnlish). Expect any dealings with the OE to be extremely costly, given the potential benefits of having them even nominally on your side.
The OE has access to resources, but not resources that you would not be able to access in other parts of the world. It has access to iron and copper for certain, but that's a result of the territories within its influence. I shall not say more than that.
Mountains? No. Small hills, I would accept. Six mountains is a range, and would really need to be Hilly territory at least.