Hmmm... there are a couple ways to go about it all I guess.
One way would be to take a tileset where the buildings are close to what you want, and modify that tileset.
Say for the water tiles, call them chasm instead, and remove the water plane... and adjust the wok object to prevent walking, a water surface of type 6 WILL allow you to walk, while a type 17 (deepwater) won't. Or you you could change that surface type to a type 7, which is a no-walk surface.
Much of that could be done fairly automatically. Getting you a working set, without a great deal of effort, and giving you a base to work from. Once you have that done, you could deepen the water tiles, to make them appear deeper, change the texture on the ground (under what was originally water) to some other texture etc...
I can and do create NEW stuff, but about 80% of the time, I start with some tile/group/set that someone else created, and add/modify as I want.
3dsmax is not intuitive to me, and the NWN engine constraints make it even harder for some things. Using NWmax, Veltools and Velmar's tileset creator, all make it easier, but you still have to CREATE things, and the way 3ds does that, makes it a bit more difficult.
I know I hate that I am limited by the default shapes that 3ds allows you to create, IE box, pyramid, plane, etc.. but just about anything can be created from those simple objects. One of the key things I have learned is to raise the default division counts a bit.
IE a plane defaults to a single division per side. That limits you a great deal, and NWN needs vertices at 125cm intervals on a ground plane to allow you to paint two tiles next to each other without visible gaps.
Anyway, take your flat tile that you just created... assuming it has those vertices at 125 cm intervals, you can duplicate that tile now, rename the duplicate to tile xxxx_a02_01 and start modifying it's shape.
Select the base ground plane, then select vertice mode on the modifier stack.
Now, your vertices show up as little blue dots (you will grow to hate them I expect, as you get more into this whole tile creation thing, and having to chase down those little blue dots...). Anyway, select a few of them, and drag them down, under the main plane. Now you have a "ditch/chasm". Not very realistic looking, but the more you adjust things, the more skilled you will become.
This brings up an issue that you need to keep in mind. For two flat tiles to connect in game to each other, they can be the same, or as in your screen shot, just one tile. For a tile that has a different shape, in this case a chasm/ditch, you need TWO tiles to make it work. One where the chasm crosses the entire tile, and one where it ends in the middle, leaving the flat part on the one side. So, now your set has 3 tiles total.
If you want to have a large chasm, one that goes completely across the tile, and does not go back to flat level on the other side, you will need more than 3 total tiles. You have to create one that is flat on one corner, and chasm on the other three, one that is flat on two corners, and flat on the other two, and one that is flat on 3 corners, with chasm on the final 4th corner. So, that means you have 5 tiles now, instead of just one...
Flat - all 4 coners
Flat - 3 corners, chasm on 4th
Flat - 2 corners, chasm on other two (on same side of tile)
Flat - 2 opposite corners, chasm on other 2 opposite corners.
Flat - 1 corner, chasm on other 3.
5 tiles. That is without having any variations on those tiles.
Variations are where say, you have chasm on half the tile, flat on other half, but you change the shape of the edge between flat and chasm... First tile xxxx_a02_01, might have a straight edge, while tile xxxx_a02_02 might have a curved in section, where the chasm goes slightly further across the tile into the center...
Anyway, I can create some basic images for that if you need it.
There is a basic naming convention used on tilesets.
xxxxx_a01_01 <-- where xxxxx is the 5 character name of the tileset. ttr01 for rural as an example.
ttr01_x01_01 <<-- where the "x" is the character chosen to represent the basic terrain, this letter doesn't matter all that much, but I try to follow Bioware basic naming to help me keep track. In this case, using an "a" would basically mean, this is the first terrain type in the set. In your case, I would assume "ground" which might mean grass, or cobble or stone. You would want to only use the letter "a" for a tile that is of that specific terrain type.
ttr01_a01_xx <-- where the "xx" is the variation. Most tilesets have at least one variant for each tile created, many, have many many more variants.
Next terrain, in your case chasm, would go with the next letter so your tile name would be:
ttr01_b01_01 for the first tile. Notice the change from a01 to b01.
Where two different terrains meet, you should pick the first letter of the first terrain, and the variants of that same tile that connect to the other terrain. ie a01_02. instead of b01_01. (leave the b01_01 as the base of that chasm terrain).
It can get real complicated, but once you get a feel for things, you will understand why tiles are named the way they are.
Groups, typically use much higher letters, r, s, t, u, v, w, x, y etc....
Crossers, like fences, streams, roads, typically start with letter g, though j or k, depending on how many different crossers you create.
Anyway, once the Harvestmoon website is back up, which should be very soon (damn isp's) there are lots of tutorials/information things regarding tilesets located in our custom content forums, the link is in my sig.
