Like I said, flat space. Two angles of approach for enemies and magic bolts. If you've got at least a gold sword, it's probably doable with a little potion spam. Anything beyond that and it's pretty tame. Even the Ball o' Hurt is enough to put down packs of Goblin warriors at a time.
Honestly I think ranged attacks are the only thing that's going to counter the flat space advantage. Your own base is a royal pain in the ass to defend against the casters, so you minimize their threat by taking away their teleportation advantage on the flat ground. Arrows and spikey balls you can't destroy with your sword will make fighting on a perfectly flat space a losing proposition, and encourage you to use something with actual terrain during the fight. And it will make the dozens of weaker goblins actually threatening.
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On an unrelated note, the Underground Jungle is probably my favorite zone. So many little things going on it, from the thorns to the roses to the light plants. I like how it's cavey but cramped and slow going, compared to flying through the open cave networks above. Too bad you can only get the Jungle Grass seed via hacking, I want to totally start growing the jungle elsewhere. Found a little gold house in it, just like up on the islands. I'm hoping for more of that type of stuff in the future (hopefully more interesting than a gold brick house and a chest....)