alright so since you are new to mapping i'm going to go over the entire map and point out some important concepts for creating koth maps, or maps in general.
first, a few issues with the entire map.
your map is extremely flat. large, flat areas are bad for a number of reasons. first, they favor certain classes over others. soldiers love flat areas because its very easy to hit splash damage, and snipers hardly have to adjust their aim when they are aiming at players moving on flat areas. classes like scout, which rely heavily on movement to avoid damage, cant stand flat areas where they dont have the ability to dodge, and shorter-range classes like heavy and pyro are sitting ducks in areas that give them no options for cover.
flat areas are also bad because they are boring to play in. movement is an incredibly important part of tf2, and being able to maneuver during a fight to gain a small advantage against your opponent, by doing something as simple as ducking behind a rock or climbing up a small hill, helps create interesting fights. when two players are competing over the advantageous position on a map, it creates conflict and forces people to fight each other. in large flat areas, that never happens.
another issue with your map is a general lack of routes. a lack of routes is bad for very similar reasons that flat areas are bad. certain classes excel on maps with many twisting routes (classes that like to flank unaware players like spy, scout, and pyro) and on a map without side routes or flanks, those classes struggle. Another reason is that it is just less interesting to play on a map without a variety of routes. when your entire team is funneled to the point through a small number of points with similar walk times, rounds are very likely to play the same way every time. this is obviously not good for the longevity of your map.
your map suffers from serious scaling issues, everything on the map could be reduced by 25-50% in size and be appropriately scaled for players to use. this pairs with the last point, if you reduce the size of the areas you have currently and add more areas around them, then you will have both an appropriately scaled map and a more interesting layout.
if you need help with scaling, try putting a well known prop into your map. the badlands spire, a flatcar container, or any prop that you are familiar with. when you put one of these into your map, it helps create context for your scale, and you will realize the areas within map are actually much larger than you realized.
a final issue with the general layout of your map is a lack of health and ammo. you have a single medium health kit, and a single medium ammo pack on each side, clumped together. if a player wants health or ammo, they will have to wait around on that pack or run all the way back to spawn.
thats it, huh.
your spawn area has a number of significant problems, but problems that are fairly easy to solve. first, your spawn only has one exit. this is a MAJOR no-no, no matter what type of map you are creating. a single demoman can completely lock down the entrance and prevent a team from leaving their spawn without having multiple players die. second, you have a huge empty space that is not useful in any way directly next to your spawn. you can easily kill two birds with one stone by simply putting an exit from spawn into the empty area, giving it a purpose and allowing players to exit their spawn through a second door.
a third problem with your spawn is trickier to fix, but much more important to solve if you want your map to be fun. when a player leaves spawn, they only have one path to move along for several seconds before they can make a decision. this can enable more spawncamping, but more importantly players
like when they can make decisions about where they want to go fight. look at a map like viaduct, where you walk out of the spawn door and are immediately presented with 3 routes. the same applies to lakeside, suijin, highpass, etc. etc. in fact, most koth maps in the game give the player options immediately upon leaving spawn.
now the point. having the point in a box on the ground is generally a bad idea, because it makes retaking the point much harder. the team that has the cap will be incentivized to sit on the highground around the point and kill anyone who tries to cap, forcing the other team to try to approach them on the highground. unfortunately due of the lack of routes, there is only one entrance to the battlements and it is easy to spam out.
your map has 2 existing entrances to the point, one through the yard and one through the battlements. these two entrances are fairly well balanced. the battlements entrance gives a huge height advantage but has a single, narrow approach that is at a height disadvantage, making it dangerous to get to the battlements but powerful once you are there. the other entrance, by comparison, is on the low ground, but has much more space in the approach and has height advantage around the entrance, meaning its much easier to get through the choke. this is good, and i encourage you to try to keep this concept.
finally, i'd like to address this question
Well, what do you suggest what I put in those wide open areas? It kind of stumps me there.
considering that figuring out what things to put in big empty spaces is the entirety of map design, there is no easy answer for this question. however, my honest attempt is this: COPY. copy from your favorite tf2 maps, copy from maps in other games, copy from real life. there are hundreds of thousands of examples all around you of successful design, and studying those things, implementing them into your maps, and seeing why they work or how they are made is the best way to learn how to make things on your own.
sorry if this post comes off as harsh or overly critical, im just trying to help! hope this can help you think about some important concepts and maybe improve your map!