I've tried a couple of different entrances, but I find that I like going through a cliff for my entrances.
I use a 3 wide hallway for about 6 squares which leads directly to barracks.
Behind the barracks is the trade depot (yes, the merchants go through the barracks room to get to the trade depot.
Next to the depot, I have my finished goods stockpile along with some secondary workshops (jewelry/clothing/leather/craft to improve the goods), and the main stairs to the rest of the fortress.
I place a drawbridge in the tunnel, just before the barracks, with some animals restrained near the entrance to act as guard dogs.
Then I litter that hallway full of weapon traps, (and one line of cage traps just before the bridge just in case). This is usually finished before the end of the first summer.
I then build a zig-zagging 1 square wide secondary hallway that's filled with traps that connects the barracks with the outer end of the initial tunnel, so when the main draw-bridge is closed people can still go in and out, but has to go through a tunnel and encounter about 10 times more traps than the short main hallway. The hall is usually finished before the first caravan comes, but maybe not quite full of traps yet.
When I have more time, I tend to build some sort of fortifications just outside the entrance and have some surface crops walled in as well as an outdoor statue garden on the roof of the gatehouse.