My character needs to have a lair in the middle of a big city, but for reasons beyond the scope of this question, it's important that nobody but him be able to get inside. This lair will be established at low level (the game starts at level 3) and is intended to be maintained and upgraded as I advance in level.
The city does not have a conveniently labyrinthine sewer system; it's likely that NPCs will be near my character's lair frequently, and they might actively attempt to enter the lair. How can I make sure that nobody breaks into my lair, with minimal expenditure of resources on my part?
My DM is pretty strict — he does not allow the creation of custom magic items, and he usually doesn't allow sourcebooks other than the PHB.
I don't have a specific character build in mind, but lair plans that don't require a specific character build are better than those that only work for a certain class.
Some NPCs might be around the level of my character, so I might need to use a range of solutions — one plan that I can implement at lower levels, and another plan that can keep out even high-level trespassers.