The Church of the Holy Apostles sits a little west of the busiest sightseeing circuit, which is part of its appeal. From the outside it is a compact late-Byzantine building of patterned brick, domes and arches, set in a small square near the old western walls. It does not announce itself as loudly as the Rotunda or Hagios Demetrios, but it rewards anyone interested in the city's Byzantine layer.
Inside, look for the surviving mosaics and frescoes, especially around the narthex and upper spaces. The church is one of the monuments included in Thessaloniki's UNESCO-listed Paleochristian and Byzantine group, and it makes a good pairing with Panagia Chalkeon and Prophet Elijah if you want to understand how much medieval Thessaloniki still sits in the modern city.