I have played siege and i assure you that it's the most difficult mission of all campaign.I have played at Tough and i can say that stopping Dreadlords is nearly impossible because they have ships too much powerful for yours.You should try to invade sometimes one of their planet to gain advanced technology, with dreadlords use the tactic of scorched earth using mass driver on planet conquered because it's very hard to retain it on the long run.Research weapons,speed and range technology with the aim only to take drengin planet if i'm not wrong is teth.Clearly you should conquer one of Yor planet first but it should be an easy task if compared to contain dreadlords advance.I hope that my advise could help you.

You can beat the misson pritty easy if you just focus in on Captureing the target planet, the far end of the map. Once you have it you win.

Here's how I beat Siege at normal difficulty.

I ignored colonizing any planets and decided to concentrate on the 3 I began with. I gave to colony ship I began with to the AI. The object is to capure that single planet in the NW corner of map so ignore everything else. Get a starbase relay on the south edge and SW edge of map. Have two starbases on the west edge of map which should be left alone for a while. Research Adv Troop Pods, Impulse Engines, etc. Build some fast attack boats on Cargo hulls, as well as a couple of AWACS sensor boats. (Fast attack boat will have 1 beam weapon and 1 hp, but it's important to make it fast. 10 speed is good, more is better. You will notice that the Dreadlords don't escort their troop transport. So as soon as you sensor boat spots one, take your fast attack boat and kill it. You might lose the ship to counterattacks but you can build more and a losing a ship is easier to swallow than losing a whole planet.

You start with a lot of cash so use it to buy factories and start churning out constructors, Fast attack boats, and a coouple of sensor boats. Ignore all the planet improvements except factories and a research center (and perhaps a entertainment center to keep your approval up.) I recommend a farm on each planet once they get to max population.
As you tech increases, build some medium hulls with plasma weapons (don't research this someone will trade it to you when you need it, if not go with laser V) and some speed.You will have to sneak these ships and the Advance troop transports around the bottom of the map and up the left side. Try to get a massive force up before you start the final push. The planet you want usually has 3 starbases around it, so take them out so the planet's ships don't get those massive bonuses.
Try to do this all before you run out of money. And have fun, it's not easy but it's Galactic Civilizations.

Well, I pretty much used guerrilla tactics against the dreadlords. You need to extensively use scanner ships (cargo hulls filled with scanners, they should move at least 4 parsecs/wk) and make a ship with 1 laser with maxed engines. These two ships should be combined to take out any dread lord transport. Make a couple of these ships and station them at your perimeter. It's gonna take a lot of fancy maneuvering to evade dread lord ships. I suggest you zoom out when you move your pieces.

You will still need to build a couple of attack ships with maxed lasers to knock out Yor and Dregnin fleets defending their planets. But one of the things I actually did was lure a dreadlord ship towards the Yor and Dregnin space and let the Dreadlord wreak havoc on their defenses.

I used one of my fast Striker ship then slipped thru a Yor-Dregnin blockade making sure the Dreadlords were following. Those Dreadlords were so bloodthirsty that they immediately forgot about my lone Striker and annihilated the Yor and Dregnin defenders. Then I escorted my transports (extensively using scanner ships) and took the Dregnin planet easy.

I beat this first on Beginner using a rather lame strategy by sending my initial colony ship and constructors straight to the world south east of the Yor's homeworld (dropping bases with the constructors as I ran out of range) this led to a very quick win.

Playing on tough was.... hmm... tough! Eventually, the way I beat it was to buff up my soldiering ability on the race selection screen (gives a warm feeling to kick the DL's ass when they invade your planets) and to religiously trade with all the other civs. Even the Drath lasted a long time and succumbed after having recaptured their lost world enough times to have picked up some serious techs which they parted with for ridiculous sums. As all the other AI got tougher and tougher through my constant trading around we started to beat the DL's back. I had some very nasty ships by then and was running a loss for about 75% of the game, only staying afloat through tech trading.

The other AI's kept doing stupid things. They would take a planet of the DL's only to lose it a couple of turns later. This did have the positive effect of them retaking it and stealing more high end techs for us to abuse the DL's with. When I finally got myself into a position to start taking their planets, I ensured that each colony was thoroughly destroyed by using the harshest invasion tactics, decomissioning all improvements and destroying the colony. The good old salting of the fields "tactic". By doing this, the DL's were slowly left with only their own system (and a huge pile of ships.... no really, a huge pile). The DL's went down and I was free to pursue the final planet.... except the Drengin surrendered the next turn!

I am fairly confident I could beat this now on any level. There are multiple ways to do it, but using your allies is such a spirit warming exercise!

I got very lucky in that mission, I quickly colonized all the planets and built up my economy. Then the dreadlords conquered one of my planets, I managed to conquer it back and then I got a nice piece of technology : Black Hole Launchers. That made the rest of the mission very easy as I was now on even terms with the dreadlords. I just built some fast ships, equipped them with two black hole launchers and the dreadlords were toast. After that I remembered that the Drengin were the target, but they didn't stand a chance of course.

I had a lot of trouble with this mission, too. I settled the planets in the nearby system, which were soon invaded by the Dread Lords, but I didn't really care. I just had two of the planets in my original system pump out lots of ships, which I sent towards the end planet, through a roundabout route to try to avoid the Dread Lords. I built two types of ships. Both had lots of life support and engines. One was a troop ship, the other had a couple of guns in it. Then I just streamed them towards the final planet. Once I could get decent ships out, I was running 100% military spending. I lost a lot to Dread Lords, but the high speed of my ships helped (I think the speed was around 7 on them). Eventually I managed to win. Putting a farm or two on each planet helps you survive an invasion. Also, kicking out a ship every couple of turns let me intercept a few incoming troop transports.

you lost @ sige LOL

wait till apoc