Necropolis (D12) has a lot of permanently locked doors. Buildings are one of the terrain features of the city, so there are doorways everywhere. Many of them have nothing significant behind them. Others are locked for a purpose, like until you find the necessary key. Necropolis is designed so you have to work your way around the city to the North End until you kill the boss. After that, you can go through the city's middle.
As for the boss, that's Lord Dire. To reach him, you require a summons that must be obtained from an NPC within one of the buildings. Keep trying the doors until you find the NPC.
Tip: When you finally make it into the town hall, don't run in to attack Lord Dire. Use ranged attacks until you get him to come to you, because if you run in to fight him, you'll spawn a great number of his allies. That's actually good advice for most of the dungeons. If you charge into battle, expect to be ambushed from time to time.