Easy, waterfalls with bridges to syphon off water into storage tanks...
Anyway, my impenitrable fortress:
This is simmilar to Nilocys design but its just coincedence
Im going to start in the plains with a large river running through it then follow these steps:
1. divert river and build stone aboveground work town.
2. dig huge ass hole as far down as I can go without too deeping or whatever
3. construct three towers to my own design with rewall and the stone I have quarryed, one shal have farms and storage, the seconds shal have manufactureing and the third will have residential and the safehouse.
Each tower will be conected to eachother and the land around with draw bridges and have a small balista/marksdwarf bunker mounted on top.
Each tower will also have the bottom most layer mined to form stilts, this will alow water to flow under them and stop anyone from digging up into the fortress.
4, construct greenglass tubes connecting each tower, these will be used to move from tower to tower, at the same time build the windows, floodgates and pressuredoors.
These floodgates will be positioned to flood farms and provide the ability to flood the whole tower section by section if required. pumps in a smaller tower alongside each main tower will provide the ability to remove the water.
5. destroy above ground town and divert the river back to its origional course, this will flood the hole and help provide an impenitrable defence.
6. In the event of a seige, the dwarfs will retreat into the safehouse, if the seige is small, my army will man the bunkers and fight them off, if the seige breaks throigh this defence and somehow gets onto a tower, the tower is flooded till the invaders drown.
The dwarfs in the safehouse will have acsess to food and the pumps so that they can reclame the fortress after the seige.
Oh, each maintower will be sectioned off into three parts each, so that if only one needs to be flooded the rest of the fortress can still opperate.
About the only way I can think of breaking this place is to climb down the pumps, but that can be avoided by flooding the pumps from the bottom and then pumping lake water upwards