Perrin cleared that out with his Asha'man. He said the Asha'man could not open any gateway, not for Travelling, not for Skimming, from within the radius. It blocks the making of gateways inside the radius, and gateways made outside to within the radius won't work either.

Moridin's comment ("don't try to trap me, I know the key" implies the key is a weave. There's a weave you can add to that for your gateway which doesn't affect Travelling but as for effect to by-pass the effect of the dreamspike.

It seems likely the dreamspike creates an unnoticeable distortion of the reflection of the Pattern in TAR. All we know about TAR must make us conclude the real world isn't affected by the dreamspike directly, it's in TAR something is different.

The TAR reflection of the area is altered, it no longer perfectly corresponds to the reality in this area in the real world, and the result is that Travelling and Skimming, -which we can surmise from Egwene's comments on Travelling involve TAR to function - no longer work. The mind of the channeler imagines his end location much the same way as you do for shifting in TAR and includes this information intuitively in the gateway weave, and it appears a gateway connects two locations using TAR. In other words on both sides of a gateway you find the real world and the hole between them, through which you can pass, the gateway itself, is TAR/a slice of TAR. With the gateway weave, in effect, your force TAR to connect two locations, or to create a hole from one to the other) . To skim you don't need to know your departure point well, because you rather go through TAR to some dimension (rather than a physical location) between the worlds, and locate your end point only. In short, the main difference between Skimming and Travelling would be that Skimming always open a gateway to the same dimension connected to TAR whatever your starting point. In Skimming, you need to concentrate to keep your platform real... just like anything created in TAR - which suggests you travel "a void" dimension of TAR on a platform you create and keep real through force of will. You then skim from one point in that dimension of TAR to the point that corresponds in that dimension to the location you wish to exit at.

Only the part of the weave concerning the arrival point varies with Skimming, the part to reach that dimension of TAR does not vary according to your starting point, it's the same exact weave wherever you are, and it brings you to a point in the dimension that corresponds to your current location. With Travelling, both parts of the weave vary, You need to weave according to your starting point, and weave according to where you want to go. You need to be extremely precise about your starting point, but you can be far more vague about your end point as long as you don't mind your gateway opening leagues away from where you intended it to open. The better you can imagine your end point, the more your gateway will open exactly where you wish it to open. That's why it's so easy to open a gateway with precision (eg: to a Travelling ground or room) when you know your arrival point well. You can visualize it perfectly.

A dreamspike messes up Travelling and Skimming because the TAR reflection of the area has been shifted/distorted by the dreamspike. Your start point (if you're inside) no longer has a proper TAR reflection, so the gateway won't open. The same way, if you're outside, your end point no longer corresponds to any place currently reflected in TAR, so you cannot open a gateway to that place.

This would also affect gateways to TAR the same way ( a gateway to TAR is a kind of "half-gatewa". Instead of using TAR to connect two locations, you make a hole to the reflection of your current location, without adding the part of the weave that lets you connect this point to another point in the real world. Instead of passing through a slice of TAR from point A to B, you simply enter TAR in the flesh).

This all explains why you can shift within the radius in TAR, and why you can't shift outside. The whole area affected by a dreamspike is distorted in an uniform fashion, and you don't notice the distortion visually. So you can perfectly imagine yourself jumping from distorted point A to distorted point B. Both places currently "exist" in TAR and are in proper relation to one another. On the other hand, if you're trying to jump outside the dome, your perception of your starting point (distorted) and your end point (not distorted) do not match, and the shifting won't work. But you can just walk from within the radius to outside the radius.

Possessing the "key", which as I said is most likely a mini-weave you include in your gateway weave, would nullify the effects of the dreamspike. Instead of using a slice of TAR that currently doesn't exist, it makes you gateway take the exact distortion of the reflection into account - allowing you to Travel to or from this area.

Theorically, it could mean a Dreamer might be able to defeat a dreamspike, but practically it's perhaps beyond human skills to learn such an area in details so well in the real world that you can then "feel" what's not reflected perfectly about it in TAR and, using will, force TAR to change to reflect the area again. Whether this is feasible depends massively on the complexity of the distortion effect. Since this was apparently used to protect areas against attacks by the Shadow (or Light, on the Shadow's side), it's likely the distortion is incredibly intricate and it's beyond the ability of a dreamer to notice the distortion in all its details and correct it. Perhaps given time it's not impossible for a Dreamer of sufficient skills to study the area in TAR and figure out the key. Perhaps it's a task Rand could give Egwene and the WO: go study the area under the dome at the BT, and figure out the key. I guess the importance of figuring out the key or use another way to defeat the dreamspike will depend a great deal of who goes under the dome during TG. If as I suspect the BT is the future HQ of Demandred to lead the Last Battle, managing to defeat the dreamspike to mount a surprise attack to defeat him might become extremely important, as other attacks coming from outside the dome, he will be ready for (the Dreadlords inside the Dome can Travel, while the attackers who cross into the borders of the dome can't. It would mean you foes ahead of you can suddenly appear in your back or at your side, or that twelve isolated enemies can suddenly can suddenly retreat, or very suddenly be reinforced by hundreds. To make it even more dangerous, it appears you can vary the area covered by the dreamspike. It would mean you can watch the enemy Travel to point A, then suddenly extend the radius to cover this area and send a huge force via gateways to ambush them. Defeating whoever is at the BT without destroying or disabling the dreamspike somehow would be a massive and costly undertaking. Isam survived to tell the tale, the Shadow will be ready for Perrin's trick (and himself was lucky Isam stupidly acted alone instead of sounding the alarm to Moridin or Graendal before following Perrin. The BT dreamspike will be much, much better protected (my hunch is that Moridin tasked or will task Moghedien to devise the protections (or perhaps a folding of TAR to hide it) for that dreamspike, and there probably will be sentries patrolling the area in TAR at all times).