After a city is successfully overtaken, the changelings will start occupying various cellars and other underground structures, because they prefer to live in the dark, and will start connecting them, building strange architecture, brood-chambers and decorations inside, correcting errors in the structure and improving stability. After a town is completely drained, the hive moves on to the next location. Sometimes they occupy nearby ruins and/or caves before launching an attack.
Even though they are aggressive, no-one knows why they actually tried to overtake Canterlot; one can only speculate that they got greedy. Ghost towns marked their way to the capital city, alerting the royal guard and Celestia to a possible threat.
The hive in general is pretty military and has a higher percentage of changeling commanders than any other hive. No-one knows what they eat to produce their building materials, but we can wager it is something icky. Their military nature is reflected in the way they shape their cocoons and other structures.
Through this military strength and aggressiveness, Queen Chrysalis has established herself as Queen Of All Queens, and the hive is the largest of all known hives. Their current whereabouts are unknown.