Painting architecture loosely can be a real challenge. Buildings need to look stable and have good form, I’ve found the best way to acheive looseness and retain that stability is by using large, flat brushes to block in the forms. By paying attention to the values and blocking in forms rather than trying to outline each building and its details, you will get a more solid, confident looking structure. Then, when it’s time to add things like windows and architectural details such as mouldings, arches and other structures, you will already have a solid base upon which to build. For the detail items, take the same approach–for example, a large flat brush can put a whole band of windows in with one “dashed line” stroke. You can turn the brush on its side to make perfectly straight window panes.
If you’re working in a dry medium such as charcoal or pastel, turning the stick on its side will acheive the same effect. I use a 1″ flat brush (soft, not bristle) for about 90% of everything I paint. I also keep a 2″ flat, and 1/2″ handy for larger or smaller works, and a 1/4″ flat for really small works, but I rarely use it. I only use a zero brush for my signature, never for actual details on a painting.