a good koth map should ideally have:
multiple entrances to mid
some way to prevent spawncamping
viability for all classes
good spawn/cap timers

thats all i could think of

Don't have a single choke leading to the point, thats basically a rule of thumb and generally means a disaster for any pushing teams.

Don't have a sightline from spawn(or near it) all the way to a point so a sniper can't get free picks without putting himself in danger of getting fragged and leaving his team at a disadvantage.
Just off the top of my head

make a map that doesn't break any of tf2's fundamental map laws
if it sucks, repeat until you get a good one
profit

-I was thinking of having three, since most KOTH maps that are played in comp generally have 3.
-I made sure there are no clear sight-lines to the spawn, and you would have to cover some dangerous ground to get there by foot.
-The stairs I added would be a good flank route, great for scouts. The high ground could be good for solly/demo (sniper also). And the choke would be the fastest route to mid, where all classes could be useful.

- There are going to be 3 routes to spawn (ideally)
- I made sure a sniper cannot see from one choke to another, with the use of 2 barricades.

Here is the first version of a floor plan I have created (the map will be symmetrical and this only shows one side!) :

http://puu.sh/9SUR8/045db2430b.png

Just to clear things up a bit, the stairs down will lead to an underground passage thing, like the one in coalplant, it would be interesting to see a flank route that could be used from just out side of spawn (might need re-balancing later)

Let me know if you have any feedback!

Sounds like you've got good ideas, and if you can get experienced comp guys to give you solid feedback things can go really well.

That being said, Hammer is a bit of an obnoxious program to learn. There are many things in it that are just not intuitive at all, and troubleshooting when you haven't wasted a billion hours in it can be awful.

Feel free to add me (hyce777) on steam if you have any mapping questions.

Also. DOWNLOAD THIS. http://forums.tf2maps.net/showthread.php?t=4674&highlight=resource

Comes with a .vmf with every single major gametype already entity'd out for you, so you don't have to go through the trouble of putting it together.

A really important point that some people overlook for KOTH maps is health and ammo spread

A good KOTH map has little or no ammo/health directly next to the point, and forces the defending team to back up or take a poor position to get medium health and sometimes even medium ammo

If you look at Viaduct, the only stuff on the point is an ammo pack and a small health that forces you into a deathtrap against soldier and demos, while all the medium health is far behind the point with height disadvantage at both locations. Coalplant has no health on the point, some ammo close, but the health requires you to back up behind your chokes or to go under, which has height disadvantage and is once again an explosive deathtrap

If you put too much on or near the point it becomes very difficult for the attacking team to push in, because the defenders don't have to give up any positioning to get health

that map is really easy to spawncamp, not just as a demoman.
there are a lot of tight doorways, and a healthpack too closely nearby.

the spawn is very similar to koth_flake, which suffers from spawncampability. see http://youtu.be/UXnh1MjhUpY?t=2m14s

look at viaduct: there are two healthpacks that have good accessibility to pushing teams and limited accessibility to holding teams. the map also has 4 ways to push mid (more if you are less general): cliff, hill, main, stairs. it also is difficult to have an agressive, near spawncamping hold. if you are trying to hold on cliff, the healthpack below is dangerous to get, and there are 3 different places you can be attacked from.
most koth maps are rejected by comp due to their limited pushing capabilities. look at koth_lakeside. the map is wide open and there are pretty much 2 viable entrances to mid. (ramps or lakehouse.) not only that, but they all fit in one player's fov, which leads me to another point. i would recommend against a diagonal koth map. not only does it make the flipping map advantage (see: viaduct) more exaggerated, it makes it easier to see more things in one fov.

going back to the spawncamping problem, imagine this:

http://i.imgur.com/9ys5ouy.jpg

or this:

http://i.imgur.com/CzilZzK.jpg

the medic is even optional. i only included it to point out how easy his escape route is. there are too many doorways and the sightlines lead too easily to spawn. look at viaduct (thats probably getting stale, but it is the only koth map people like to play in competitive) there is literally no way to look into spawn without being inside the spawn area. the entrance by cliff is obscured by the height difference between doorways and a bend in the transition room. the entrance in the middle is a turn. the entrance on the right is a turn. not only that but none of the areas' doorways look towards spawn.

the stair tunnel is a good concept, but in execution its sort of flawed. the flank area is a good distraction from the spawncamping problem, but it will likely end up like coalplant/ashville where nobody wants to enter it because its basically a death trap if you are caught, and is irritating because of the height disadvantages/advantages. and it is fucking massive compared to a normal size tunnel.

The height ground is also like the stair tunnel- it seems good in theory but would suffer in execution. this is because it is just a jump over a railing from mid, and not only that but it overlooks the main choke (which we will get to in a bit). it looks directly towards mid, and unlike the cliff area on viaduct, it faces spawn directly, and can be utilized too heavily by the mid-holders because of its shape/direction. a demoman can sticky spam that area much easier than any other class can spam down from it, because its wall directly faces mid and the demoman doesnt even have to rotate from the point to look at it. (a note: don't fix this by making the space bigger. big is bad for koth, as well as spacing, a problem coalplant suffers from. its a massive map and everything seems oversized. the houses, the midpoint, the under area, etc.)

my last point: the main choke.

http://i.imgur.com/skDVEkh.jpg

a main choke is very bad for a koth map, especially if there are only 2 viable ways to approach mid (the high ground and main choke, as seen). A main choke is just a bad idea from all points for a koth map. pushes are hard in tf2, and a main choke gives the holding team a massive advantage. one demoman spamming a choke = that choke cant be entered without an uber. and requiring to uber before you reach the point is not good for an attacking team. this, again, should not be fixed by making the choke larger, but by adding other areas to attack from. not only is this a problem with the main choke, but the main choke can be watched in the same fov as the high ground. every player exiting the spawn can be seen from the holding team no matter where the attacking team goes (unless they go to the stair tunnel, but all that will do is give them a tighter choke where they literally all will die unless the holding team is oblivious.)
not only this, but look how easy the flank can hold the only thing dragging the demoman's mindless, unmoving m1m2 spam away from the mainchoke.

god damn thats a wall of text. you probably understand my suggestions.
also dont be discouraged by this list of problems. they are all not-that-difficult of fixes, and at least you haven't started mapping yet.
i wouldn't have typed this up if i didnt care about the result, keep going

Hey Hyce, thanks for the response! One of the reasons I decided to start trying to make a map was because I downloaded cp_logjam, and ran around on it a bit, looking at all the important features. I watched a youtube video where the guy downloaded the pack so I have that done.

Hammer reminds me of a program that I have used a few times called Rhino. Very similar layout, and it is also used for 3D modeling. Hopefully experience with this program will speed up development a bit.

Thanks for all the feedback Reminixe, it was very helpful. You took into account lots of things that I didn't at first.

http://puu.sh/9TC51/8ce2908ae8.png

V2 is done, here are some of the changes I made:
-I liked your advice about the diagonal map, and looking back on other successful comp KOTH maps,all of them are diagonal. I fixed this by butting the spawn in the middle and moving around the other components.
- Choke: There is still a main choke, but this design is borrowed for a lot of other maps with "S" shaped choke points. The aim was to stop the demoman from dealing damage when spamming into choke, the health/ammo in there could sustain a team if the demoman did become aggressive and risk his life by entering the choke.
-House: I added this house to protect the team that is leaving the tunnels. I don't know if this area would provide cover, or just become a death trap because of the lack of movement room (if this was the case it could be prevented by adding sliding doors)
-Smaller Choke: This choke has no health/ammo, and is straight as opposed to "S" shaped. The goal behind this was to pressure players that could be spamming choke, or that could be spamming the house. I designed this choke with it being a secondary entrance way in mind. I didn't want it to become the main point of entry so that is why it would be easier to spam out.
-High Ground: Relatively unchanged. Added a barricade near the health and ammo which is now on the cliff, so you can be protected from most spam from the CP while grabbing it.

http://puu.sh/9TCKU/01ce56d5ee.png

-------

http://puu.sh/9TCZJ/0900fbbd9c.png

I think this covers most of the areas at fault in my first drawn up version, let me know if anybody sees any other game breaking unbalances.