You can go to many other places in FFXII 'early.' One of the few major barriers to that is high level enemies, otherwise you're not stuck down corridors with no other choice but to finish the 'level' -- which is what FFXIII's 'world' is more akin to. We can agree to disagree on how good or bad linearity is, but FFXII is one of the most open ended games in the series, while FFXIII is the most linear. FFXII isn't Skyrim or anything, but it isn't comparable to FFXIII.
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"When I was a young man, I had liberty, but I did not see it. I had time, but I did not know it."