I think in part because the two special DE detachments don't have access to them and in part because they run contrary to the Dark Eldar archetype of highly mobile, "glass cannon warfare," I have never seen a DE player use (or even talk about using) a Fortification as part of a Combined Arms Detachment.
Given the DE strengths in mobility, firepower, and flexibility (depending on their opponent, they can pursue shooting and assault strategies, unlike many other armies which really can only focus on one or the other), I believe certain fortifications can truly "force-multiply" a competitive DE list. Let's take a look:
1 - Aegis Defense Line -- One of the great vulnerabilities of DE vehicles is that they are so fragile, they almost have to jink in order to have a chance to survive. If you are delivering units to assault, that is alright, but if you are shooting, that is a major disadvantage for DE. Enter the Aegis Line.... now, your Ravagers can use their long range Dark Lances and shoot and still get (with Night Shields) a 3+ cover save. Same goes for your Disentegrator/Dark Lance armed Raiders and Splinter Cannon Venoms... for the transport vehicles, this means their embarked unit can be biding their time for a couple of turns, letting the Power through Pain build up so that when the vehicles finally do dart out from the ADL and disembark their passengers, the units will likely have FNP, Furious Charge, and maybe even Fearless. The ADL can also be a good defense for you large, 20 man Kabalite Warrior squad that is holding an objective and taking stationary shots with its Splinter Cannons or Dark Lances.
A second benefit of the Aegis Defense Line for DE is the ability to add a Quad-gun, Icarus Lascannon, or Comms relay. The first two help with anti-flyer/skimmer defense (since Razorwing/Voidraven are the only dedicated DE flyer defense) while the second helps with Reserve manipulation, ensuring the Webway Portaling 10 man Grotesque squad with Liquifier Guns arrives turn 2 as planned.
2 - Bastion -- Like the ADL, you can use it to supplement your anti-air or better control your reserve rolls. Unlike the ADL, you can't use it support vehicles, but the Bastion is still relatively cheap and is the perfect platform for a couple of squads of Trueborn with max Blasters and Dark Lances to sit in and shoot out the firing ports/over the battlements in relative protection. Bastions also let you pull the whole "Escape Hatch Slingshot" trick... i.e. say you have a squad of 9 Grotesques + attached Succubus deployed inside a Bastion with an Escape Hatch positioned 12" toward your enemy DZ.... turn 1 they can disembark 6" from the Escape Hatch, then run, giving them and effective 12"+6"+ D6" = 19-24" move turn 1, thereby setting up an almost certain turn 2 charge (do the same thing for a squad of 15 Bloodbrides or max Wracks or even allied Wraithblades led by Lelith. Finally, the Bastion lets you completely block LOS to vulnerable units, like that Venom full of Incubi or the unit of Haywire Scourges that you are waiting to jump out and counter-attack the enemy assault with, but don't want gunned down turn 1.
3 - Firestorm Redoubt -- This provides many of the similar benefits to the Bastion (including access to Quad-gun/Icarus Lascannon/Comm Relay and the ability to pull the "Escape Hatch Catapult" trick) and can provide an excellent firing point for more shooty units like Scourges or Trueborn (or even Kabalite Warriors). The advantage it has over a Bastion (and what justifies it higher price tage for a DE army) is its firepower.... either two Quad Icarus Lascannons (for very strong anti-flyer/skimmer/flying MC... a great way to take down those annoying Wave Serpents!), two Punisher Cannons (amazing anti-horde, not that DE really need this, though), or two Battle Cannons... this last one is probably the best for DE, as it fills a gap they really lack i.e. long range, high capacity anti-MEQ shooting.
4 - Void Shield Generator -- Purely defensive in nature, this can provide the perfect "shielding" (pun intended
) for your DE forces while they wait until the ideal moment to dash forward and strike... having up to three Void Shields projected in a 12" radius around the Void Shield Generator means that you could potentially have almost your entire force covered by a three Hull Point, AV 12 shield, thus enabling them to be immune to any shooting S5 and below and survive a lot of other shooting that would normally destroy them outright. If you really want to go all out with this, take the Void Relay network, which enables you ro get up to 3 Void Shield Generators (and at least one Promethium Pipe Relay, since it is required)... this will give you up to 9 Hull Points worth of AV 12 Shielding to protect your DE force from enemy alpha striking turn 1 (looking at you here, Tau).
5 - Fortress of Redemption -- Kind of a hybrid of several of the fortifications described above, for the price of a tricked out Voidraven Bomber plus some change you can have an AV14 fortress with anti-air (TLed Icarus Lascannon), anti-infantry (Krakstorm missile launcher), protection (firing ports and ramparts), and even Escape Hatch for mobility supplementation. Kind of expensive, but certainly can provide a solid "anvil" to complement your typical "glasshammer" DE strike force.
6 - Macro-Cannon Aquila Strongpoint -- If you are willing to spend over 25% of your points in a 2000 point game on a fortification, then this may be the one for you. The Aquila Macro Cannon is amazing, with a 72" range and two D strength Large Blasts a turn... manned by an Archon for BS8 (and a max of 4" scatter) or even Lelith for BS9 (and a 3" scatter max) and you can precisely "snipe" out each turn the unit that is biggest threat to your DE force.... a team of Missilesides threatening your Raiders... ID them with two super accurate Destructor Large Blasts... a group of Wave Serpents close packed together... bring them all down in a single volley from the Macro-Cannon.... Thunderwolf Cavalry bearing down on your DZ.... eliminate them as a threat in a single shooting phase... etc etc.
So, those are some thoughts on DE and fortifications. Obviously, you would want to model (and name/give fluff background) them in Dark Eldar appropriate manner, so instead of Icarus Lascannon you could have a "Heavy Disintegrator," instead of an Imperial Bastion you could have a "Commorite Bunker" etc.