What is the right word for the arches/entrances found in a park or in roof garden, where creepers would crawl? "Bower" doesn't seem to be right word.
Are you thinking of 'arbor' or 'pergola'?
Most of these bowers look natural.
https://www.google.com.au/search?q=bower&rlz=1G1ASUT_ENAU421&tbm=isch&tbo=u& source=univ&sa=X&ei=W6VnUcObA62higfO2YDwDA&ved=0CG 8QsAQ&biw=1366&bih=608