"I build the city."
Translation:Ego urbem condo.
He must have been referring to some song lyrics, e.g. the 1970's era California-based (San Francisco I think) rock band Jefferson Starship sang something like "we built this city on rock and roll" -- there's some more relevant comments elsewhere in this thread. -- Afterthought: maybe it's worth adding that San Francisco CA is noted for being earthquake prone, and the band Starship may have been alluding to that, among other things --
After finishing the course, it looks like it depends on what the definition of "with" is... Is it a person or people? ("I built this with the help of my friends")? I think my cum+[ablative] seems to imply "rock and roll" are people who helped me. Is it a tool I used to build ("I built this with a hammer and nails")? Then I think it just takes the ablative, no preposition. Is it the raw material it was made from? ("I built this with the leftover bricks from my other project"?) Then I think that's ex+[ablative]. Anyone have Dennis Lambert, George Page, Bernie Taupin, and Peter Wolf's e-mail addresses? We need to ask them exactly how they built that city with rock and roll. (And Heaven help us if Blue Öyster Cult come along and set our Cities On Flame With Rock And Roll!)
Silly me, I should have listened to the song before going off on this tangent, and realized the lyrics were "We built this city on rock and roll," not "with."
Well, I suppose there's worse ways to learn prepositions and cases than taking a silly joke too seriously...