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