Work has started on pl_vertical, a payload map for tf2 where the cart needs to be pulled up one shaft and then lowered down another. I'me finding that vertical maps can really add a lot to gameplay that just isn't possible through horizontal maps. Of course, it does require a lot of new stratagies for the player to understand whats going on and be able to comprehend the map's routes and his options.

From my post on Interlopers.net to another member working on a vertical payload map:

I'm also working on a vertical payload map (pl_vertical), I have a first section in blue that is industrial and as the cart is moved to the red base when it starts moving down it becomes more thrown together. What I've seen so far is that to make it balanced you need lots of side routes and alternate ways to get up else the team defending will have too much of an advantage (Being able to shoot down on them and drop down on them), however using side passages and tunnels to even this out makes the map very complicated for players to grasp. official tf2 maps are fond of using stairs and ramps to elevate the player.However as you can imagine, an intricate system of side passages only using ramps and stairs would be very complicated and not move up very fast. I tried this at first but it's very complicated. What I recommend is to fall back on more tfc like designs with vents that push the player up quickly and small elevators that move fast as soon as a player steps on its surface. It may seem a little out of place in tf2 but I've found it's very useful.

When passages and routes are out of the player's vision he has trouble visualizing them making the map complicated for those who don't fully understand the layout. In upwards based maps this is troublesome because realistically exposed routes upwards are somewhat rare. It's important that the player is able to clearly think where passages are. First of all this can be helped by using large cavities that house lots of routes within them rather then just lots of interconnecting tunnels, that said interweaving tunnels in one big room will also confuse players. I've found that it's best to have the alternate routes out of the way, but clearly outlined. Say a tunnel or elevator shaft next to the main room but with gaps in the shaft so that its route can be followed with the eye from outside. In the red's style of building this is easier to do, but in the blu's more structured style it's somewhat hard, because you are doing a single stage map I'd recommend focusing more on red's architecture as if it took place in a red base.

It requires a lot of planning but vertical maps can afford the player a lot more freedom.