Traces of the citadel can still be seen today at many sites.
The main site evident to tourists is the Northern Gate. French colonialists of the 19th century systematically destroyed most of the citadel (a criminal action by any measures) but preserved the Northern Gate because of clear evidence of damages caused by artillery shells fired by their Navy from the Red River nearby. The battle scars are reminders to the subjugated populace of the superiority of French firepower and technology.
The gaping holes are shown in the photos above, taken at night, June 29, 2009.
Wikipedia in English provides little in regards to the Hanoi Citadel. If you read Vietnamese, click on the blog title for a more thorough write-up by Wiki Vietnam.
The locals call the Northern Gate, của bắc or bắc môn or more simply thành Hoàng Diệu to commemorate the commander/governor of the citadel who valiantly resisted the French attack of April 25, 1882, and paid for it with his life. The attitude of Tự Đức, the emperor of Annam, towards French repeated violations of Vietnamese sovereignity in the 19th century, is nothing to write home about, a very sad chapter of Vietnamese history.
Wiki Vietnam also provides a fair amount of information about Hoàng Diệu : http://vi.wikipedia.org/wiki/Ho%C3%A0ng_Di%E1%BB%87u
