The main gate should always be open. If you want to get into the first part of the city, ask the guards and they'll let you in if you're a dwarf. The other halls are closed for Non-citizens.
Like Eisenbein wrote. You can talk to the guards beside the gates in. German "Schwester Tor auf bitte" for examble. You can also talk english at the silverbrand forum if you have questions, the most of the dwarven-POs talk english too, but there is normally no need because there are less english speaking dwarves.