I dug a long tunnel to the north and stopped before gold blocks, than dug a horizontal tunnel that prevented any enemy keeper to enter further to the south (so to my dungeon), dug a 5x10 room right before the wall and placed there about 8 Storm Vortexes later, 2 Bone Chillers, 2 Wells of Souls and of course 3x3 Garrison.

It was also important to dig westermost corner for gold shrines and eastermost corner for Foundry and creatures there, however I had to put 3 SV somewhere along the path to these places because enemy underlords likes to put underminers behind any walls (at least it was only this one underlord to the right).

After placing all the traps to the north and having quite well trained troops (4-6 lvl) I made a wide open breach there and let all enemy creatures in to my dungeon, just to see them being obliterated by traps I've put there and some of my creatures. After many of enemy creatures died I immediately rallied my troops to one of enemy dungeon cores and than onto the second.

How do you build so many traps with that little lockable mana? I need to create more workers, and end up with one or two traps available.

As more sins you unlock as more lockable mana you get, get them fast as much as possible. I don't remember if there were mana-giving constructions (50 mana per 1 item), but if yes build some of them in later game. Use spirit workers from Alchemy Lab for bigger excavations or tile claiming and have no more than 8-10 "normal" Workers or just make many workers for excavations at the beginning and after that remove some throwing them back to Dungoen Core. Build traps with repair traps spell and/or by placing toolboxes from Foundry using hand - don't allow your workers to build defences - you actually need workers only at the beginning of the game, later you can throw them all into the abyss. Try also to fortify walls via ritual.