(It's broken.)

One of the most distinctive features of the symbol of my home town is the way the outer wall does not run all the way around. So, was this an ahead of its time piece of asymmetric architecture?

Sadly not. A massive earthquake in 1349 caused the south side to collapse. The resulting rubble was carted off to build other buildings, just as some of the interior stonework was. Naturally the marble that once formed part of the building was also looted.

Some may be inclined to think that the partial wall was intentional given how straight the slope is, but these are modern reinforcements which were designed to strengthen the walls. The original wall was symmetrical and ran all the way around.

Thus we have one of the more distinctive "views" of the Colosseum; that of the split wall.