Motorists can be impatient with pedestrians, so take extra care when crossing the road.
Drunks in Seoul tend to be better behaved than elsewhere, so walking around at 3am shouldn’t pose a problem. There’s always an exception, of course, so it’s best not to antagonise people who have been drinking.
Police in full riot gear, carrying shields and batons, are a not uncommon sight in downtown Seoul. Student, trade-union, anti-American, environmental and other protests can occasionally turn physical. Keep well out of the way of any confrontations.