He can - and it appears to be a real window.
It is apparent that they are not, in fact, on the ground floor of the tower.
Outside they can see a grand city, stretching for miles, its boundaries marked by a ward dome - the source of that prismatic light. Even from this height it's easy to see what Jaim meant by the cities being works of art - the architecture is all soaring, elegant, pale stone - the actual colours obscured by the light of the dome. Plazas dot the city here and there, several leading up to large buildings presumably meant for important persons or services. Above the main streets and sparkling canals are high walkways, allowing passage from the higher doors of the various towers.
There are some number of wild, overgrown patches of land dotted here and there, the remains of parks and gardens grown out of control over the past three centuries. The vegetation hasn't left their bounds, however - presumably the Artefact in control of the city has some way of penning it up.
Just barely through the wards they can see solid stone. The city appears to be buried - but thanks to the wards it is still, miraculously, intact.