One of the most popular buildings to reduce is the academy. However you need this at level 30 to run 3 of the 4 culture activities. You also need it at level 20 to produce colony ships.
Slight correction, it's level 22 for production of colony ships.
I have to agree with most of the rest though. As for the wall, that should vary depending on the type of city it is. In offensive cities, you don't want to be leaving your troops in the city to defend, so a wall isn't required. I keep a wall at level 6 in my offensive cities, for temple construction. In defensive cities, obviously you're more likely to want to keep your troops at home to defend against attacks, so a wall is of more use. 20-25 is usually a nice level to have it at in a defensive city, but I know some people choose to take it down to 10-15, after they've built their Tower.
Once you get a decent number of cities, keeping high level resource buildings isn't essential, as you should be able to push resources from your other cities, to rebuild. Equally, if you're a gold user, buildings such as the Senate, Barracks and Harbor aren't required at a high level, as you can speed up construction/recruitment with gold. I usually tend to lower my resource buildings as far as 10-15, but sometimes it's best not to go too low, to allow you to rebuild other buildings if hit by Lightning Bolt (level 15 timber camp and level 10 silver mine, for Harbor rebuilding, level 12 quarry for Temple rebuilding).
Temple is personal preference, really. I like to keep Temple levels high, but once you get a decent number of cities (30-40 fph) you can start lowering the levels a bit. Marketplace is, again, personal preference. I like to keep high level Marketplace in most cities, so that once all farming villages are maxed and I cannot train more units, I can send resources to other cities instead of having them stack up in my warehouse, unused.
