You could translate with “witness of the glory [..] as well as the transitions […]”.

“tant” carries here the idea of continuity and also an idea of accumulation of events.

The meaning of “tant” here is “autant” or “tout autant” which means “equally”. As a comparative term, “que” is attached” to it. Copro’s translation looks correct to me.

On the other hand, “de” is attached to “fut le témoin” (“être témoin de” means “witnessing”).

It is hard to translate word for word into English, because it follows a structure (“tant… que…” → “as much… as…”) usually, but not in this case, easily translated as “both A and B”. Here, A itself is split into two distinct events, that is the glory and the fall of the Tokugawas, making the statement of B, the transitions in Japan’s history, appear a little redundant.

Something like the following, though it is not very elegant:

Nijo, a castle that has been the witness of both the glory and the fall of the Tokugawas, and the transitions in the history of Japan.

Perhaps switching “that has been the witness of” to “that has witnessed” would be a little more native in English (?)…

The solution proposed by copro is certainly a good option to make it more elegant, though it weakens the statement a bit, not putting so much emphasis on the extent of time and radical changes society went through during the existence of the castle:

Nijo, a castle that has witnessed the glory and the fall of the Tokugawas, as well as the transitions in the history of Japan.