One of my weaknesses as a DM is probably battlefield design. I often forget about this completely, and have to make up a map on the spot when we roll initiative. I try to avoid this before major plot battles, but since I don't explicitly design my battlefields before-hand very often, I'm not sure how to handle it.
- I'm often unsure of how to use elevation differences and hazards to make things more interesting, or to favor one party over the other.
- I have trouble balancing the size and openness of the battlefield to both allow the party to move around while at the same time requiring that they engage the enemy.
As an example, let's talk about the penultimate encounter of my campaign. The party will be fighting the Skeleton King created in this previous answer. I don't want the quality of the encounter to rest on the quality of a map I make up on the fly.
This specific encounter will be in the ruined throne room of Castle Never, from the Neverwinter Campaign Setting. Castle Never is described as a "big, hulking ruin" that was largely destroyed in the cataclysm. The cataclysm was a combination earthquake and firestorm after a volcanic eruption prompted by a fire primordial having a nightmare. The cataclysm "struck it hard, toppling towers, collapsing walls, and starting fires that burned throughout the structure".
As far as the hazards go, this area has been affected by the spellplague, which alters reality in strange magical ways, so they can be pretty crazy. Bubbling liquid bricks that float upward, coming from nowhere, distortions in gravity, weird sounds and lights, alterations to the functioning of magic, whatever. There's also a lot of undead activity and necromantic activity in the area, as well as some fungal growth from a nearby colony of myconids.
For inspiration, I'm looking in part to the original battles with the Skeleton King from Diablo I and III. You can see the battle from Diablo in this video (actual encounter starts at 2:15). The fairly similar battle from Diablo III is in this video (actual encounter starts at 1:00). The surroundings in III are closer to Castle Never's throne room.
In both of those fights, I think the battlefield is too open. The players don't really have to engage with the skeletons too much, and can mostly kite the king.
I'd like something a little more claustrophobic, like the fight with Queklain in Final Fantasy Tactics. Here's a video of that fight (actual encounter starts at 1:20). I like how the raised area in that fight makes a big difference in positioning, despite being fairly simple.
It would also be neat to have some differences in elevation and perhaps hazards, caused by the ruination of the castle. This should favor the king by making it easier for his skeletons to mob the party, as well as making it possible for him to hit multiple characters with his special attacks. Several of the party members have abilities that allow them to shift themselves or allies, and I'd like them to have to take advantage of that in order to overcome the battlefield.
The party consists of five level 10 characters: a human Cavalier (Paladin tank with a halberd), a kobold Invoker (lots of shifts and forced movement, with radiant damage), a genasi Warlord (Int-based tactician that grants attacks), a changeling Executioner (Assassin with both hand crossbow and rapier, multiclass Warlock), and a half-orc Slayer (charging spear fighter, my GMPC).
The party has excellent defenses with good attack ratings. They can do a lot of radiant attacks, and the paladin is good at stunning enemies. The Invoker and Warlord can both move their allies around the battlefield, and I think almost everyone has an ability that lets them shift multiple spaces.
The Executioner can climb walls like Spiderman, and the Slayer is an amazing jumper. The Warlord can fly.
What approaches can I use to address the issues I have with encounter design?
So, in the context of the example encounter, please explain how I can create a battlefield that favors the skeleton king's attacks, forces the party to engage with his minions, and gives the party ample opportunity to take advantage of all their movement skills?