Yep, that's was my main intent.
How do you keep them from hitting the pressure plate again to close the hatch?
Will building destroyers tear down a floodgate during their attempt to leave the map?
I don't bother. Here's my layout. The area to the left is "uncontrolled" and towards the right leads towards my fortress.
Z = surface of map. Not going to show the surface since it should be fairly obvious given the Z-1 level...
Z-1 = 1 level beneath.
▒▒▒▲+▒▒ Entire map edge trenched like this. The ramp/floor combination allows wagons to pass unhindered (for caravans) while those on foot
▒▒▒▲+▒▒ have to travel around the trench until they reach the "welcome center" The reason the border is 3 tiles instead of 1 is to avoid issues
▒▒▒▲+▒▒ with trees and such that blocks caravans.
▒▒▒▲+>▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▲+▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▲╔╗¢^╔════════╗¢^^^^^^^^^^^^^^^^^¢▲▒▒▒ Each ^ next to a ¢ is a pressure plate triggered by non-citizen creatures, any weight.
▒▒▒▲║║▒▒║++++++++║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ The other ^ are weapon traps. For this diagram, there are a total of 7 pressure plates,
▒▒▒▲║║¢^║++++++++║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ 7 hatch covers, and 15 weapon traps. Additionally, there is a 2x9 drawbridge
▒▒▒▲║║▒▒║++++++++║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ and a 10x10 drawbridge controlled by levers inside the fortress.
▒▒▒▲║║¢^║++++++++║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▲║║▒▒║++++++++║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▲║║¢^║++++++++║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▲║║▒▒║++++++++║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▲╚╝¢^║++++++++║>▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▲+▒▒▒╚════════╝▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▲+▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
Z-2
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒<^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^▒▒▒ Just a lot of cage traps in a tunnel that runs underneath the hatch covers.
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒^▒▒▒
▒▒▒▒▒▒^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^▒▒▒
▒▒▒▒▒▒^▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒^▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒^▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒^▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒^▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒^▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒^▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒^^^^^^^^^^^^<▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
Usual mode of operation. Both drawbridges are lowered. When a siege with building destroyers is present, the outer 2x9 drawbridge is raised and lowered as needed to either destroy the building destroyers, or to encourage the building destroyers to take the down stairs into the cage trap corridor.
The 10x10 drawbridge is usually kept lowered. However, because I'm impatient, when a siege by numerous undead is present, I'll have a dwarf throwing the lever on repeat to raise and lower the bridge to crush the undead (I've found that the undead generally don't have any loot, so I don't consider it a waste to simply smash them into oblivion.)
The reason for 5 entrances is to help compensate for the 100 tick reset delay on the pressure plates. I wish the room to be as easy as possible to enter and I'm using 5 entrances simply because that's the most entrances I can cover/protect with a single drawbridge. There's nothing preventing multiple drawbridges from being used and having them all linked to the same lever and controlled as a single unit and of course, then permitting more hatch cover/pressure plate pairs to make the entrance even easier. Only using 1 goblin grinder as the exit to maximize the number of weapon traps that have to be jammed before the grinder become ineffective. Each weapon trap has only 1 weapon to minimize the chance of being jammed. The weapons used is a fairly random mix of menacing spikes, spiked balls, corkscrews, axe blades, etc. Preference to non immediately lethal weapons is made to once again, minimize the chance of a jam. However, I do include a few blades to deal with creatures that have to be beheaded or bifurcated to be killed.
If the 10x10 room has some creatures that are attempting to retreat (it's pretty obvious when it happens), the 2x9 drawbridge is manually closed so the creatures stop futility attempting to path over the hatch covers and instead path towards the down stairs leading to the cage corridor.
This welcome center will pretty much handle all sieges prior to flying creatures. However, kobolds are quite capable of passing through without hindrance and you should have some sort of sentinel at the entrance to your fort proper in order to spot them before they manage to steal anything. If you do manage to spot them prior to them stealing anything they'll simply run away and they can be otherwise ignored. However, if kobolds do manage to steal enough items, they'll come back and ambush you. And the welcome center will provide no protection against a kobold ambush.
I also tend to trench out over the weapon traps and then construct a floor over the trench in order to flag the goblin grinder as lit and above ground in order to prevent miasma. It would also be a good idea to check on the welcome center from time to time in order to clean it out. Some creatures will enter the 10x10 room, and then attempt to escape causing some lag as they keep trying each of the 5 entrances causing the hatch covers to open and close. When this happens, just raise the 2x9 bridge and wait for them to path through the cage corridor. Also goblin thief remains (and possessions) will be frequently found in the goblin grinder without any corresponding combat reports. (yes, snatchers can not get through that welcome center).
Some possible refinements.
1. Provide a means of flooding the welcome center with water and a means of draining the water. Useful for mass killing and dealing with weapon masters (who are VERY good at dodging the weapon traps in the goblin grinder. A weapon master from a latter siege can survive the grinder for a long long time and in order to get rid of them, you'll either need to flood the area, or send in some armed dwarves to fight 'em.
2. Replace the walls with fortifications and station archers on the other side to shoot at 'em (also useful for dealing with weapon masters).
