Downtown ham
Having lived this situation lasting a few years in Paris area, I could note the difficulties which one can encounter in a large city.
Difficult to free yourself from parasites linked to the urban environment but you can always find a solution to be active on HF bands. At the time, I used an isotron made with the means at hand, hung on the kitchen window (the only one that was not metallic). This allowed me many contacts anyway !
Some tools
For the development of these antennas, the ideal is to use an antenna analyzer or a noise bridge. Here are some useful links:
Noise bridges
MFJ202B from MFJ
RX100 from Palomar (no longer available)
Antenna analyzers
Commercial:
VA1RX from Autek (no longer available)
MFJ259 from MFJ or MFJ-223, MFJ-225
miniVNA HF/VHF (on PC).
kit:
Tenna dipper
AA 908 from american qrp club