They have so much health, and you can't slow them down at all. Literally the only way I've been able to beat any of them is hope that they get stuck on level geometry. I've used maxed out slime traps, barricades, ax traps, bolt traps,all with maximum upgrades, they are mildly inconvenienced and then walk into the portal. That is, unless they get stuck, then you can kill them. (Edited for clarity)...

I keep having this issue where the barricades don't change the path of the monsters. This is a problem because it's supposed to change their path and make them take a much longer path to get to the exit. Please fix! Thanks.

I like Defence Grid too but this is great too and has more content.

Do these map disappear if we lose them? Or only when we succeed in clearing them? I'm accumulating a lot of such maps to unlock distant levels, and I want to know if it's worth using runes to play ethereal maps, or if it's better to play it safe.

Why would anyone want to put more than one level (if even that) into bribery? Other than to gain access to Avarice? It increases the minimum activation amount, it doesn't lose the percentage Modifier even at lvl 15, and it just sucks your bank dry so Scavenge becomes less effective....