I noticed the "Zot Defense" mention when playing the online version, but as i was busy on the tournament i didn't explored that gametype and forgot totally about this after the tournament concluded.
Interesting to see this added now.
I tried and got a bit puzzled.
Started as a troll berzeker, not knowing how this thing played, i noticed in my abilities i could build "Dart traps"
So i did, took me a while to understand it was working, as i was clicking and could see nothing, in fact you can't see the traps you build unless you walk on them (trigger them) or your trap detection skill kick in.
I explored a bit and noticed the 2 monsters entrances and their paths were very separated from each other, meaning that i couldn't do anything other than sitting on the orb, as it would be impossible to stop both sides in the same time.
And once a monster start sitting on the Orb, your health goes down very fast.
Where the fun begins is when you gain levels, you can build more traps (their costs is on your experience pool), and of more different type.
At level 2 i could build Oklob sapling, but it eat 60 experience pool for one (dart trap eat only
, difficult as you lose experience pool during the fights as in normal crawl so building an Oklob sapling may be difficult at low level fortunately (or unfortunately due to their danger) you have uniques appearing that are worth a lot of experience pool at low level.
In level 3 i could build arrow traps etc..., etc...
All in all, this game mode can be very fun, too bad the traps are invisible to you when you build them, but it's fun for once to see monster running on your own traps instead of the opposite
edit :
Died at level 8 during an enormous Gnoll invasion, i couldn't see the end of that wave of them, my Oklob sapplings and later plants did some big ravaging, but my troll was hit by so many items thrown by the Gnolls that it couldn't resist for very long.