If you are booking Seoul for the first time, the winning move is not to cram in more attractions. It is to book the trip in the right order: entry rules first, then flight timing, then hotel area, then airport transfer, then only the time-sensitive activities. Seoul is easy to enjoy when the logistics are clean, but a short trip gets punished quickly by a bad hotel location or a route that crosses the city too many times.
For a Singapore-based traveler, Seoul is usually best handled as a 3 to 5 night city break with one central base, one historic day, one modern neighborhood day, and one flexible block for food, shopping, or a spa. If you keep the itinerary compact and book around neighborhoods instead of individual landmarks, the city feels efficient rather than rushed. That is especially true if you arrive tired, land late, or travel with family.