Lagos is a city built around water it rarely lets you see. Traffic hems you in at street level; the lagoon only really opens up once you get above the fourth floor. That is the whole appeal of the rooftop scene here — for the price of a drink you buy back the horizon, and for about two hours before sunset the whole city looks like it is on your side.
The practical thing to know is that Lagos rooftops split into two moods. The Victoria Island and Ikoyi terraces are sundowner places: you arrive around 5pm, the light does its thing over the water, and conversation is still possible. After about 9pm most of them turn into sound systems with a view, which is great for dancing and terrible for meeting anyone. If your goal is to actually make friends rather than just be out, go early and stay at the bar rather than taking a table — tables in Lagos are territorial, bars are not.
