I find that using a 'buffer zone' with two raising drawbridges works really well to protect forts which are unsafe or have no military; not only to protect yourself from a siege, but to protect any visiting traders as well.
The basic idea is to have 1 entrance to your fort, a raising bridge to keep your dwarves in, and then a strip of land outside your main fort, protected by another raising bridge that stops anything entering from the outside.
Like this:
Bridge Depot Bridge
[Fort Entrance] ::::|=====|::::::D:::::|=====|::::: [Outside World]
When the traders arrive they will unpack at the depot. When they're all there (wait for the slow camel people), you raise the outside bridge, disconnecting you and the traders from the world. You then lower your inner bridge, allowing your dwarves into the Depot area. You do your trading etc and raise the inner bridge when everyone's back inside. Then, when the traders are ready to leave, and if it's safe, you lower the outer bridge and allow them to leave.
(Of course, the inner bridge is entirely optional, and is only useful if you don't want dwarves wandering outside.)
If you WANT to be attacked, though (for a nice source of Iron to melt and make some armour with your newly aquired anvil), it's as easy as setting up a 1-tile-wide path at your entrance and lining it with stonefall or weapon traps. You don't even need a military. Your first siege *should* be pretty light, and you'll be okay even with just a few traps guarding the way. But bare in mind, a siege will only be beaten when about 70-80% of the attackers have been captured or killed. The rest will flee.