There are 2 golden rules to avoid mapping catastrophes:
Hammer is an outdated tool that is prone to crashing. There are also many issues that can be caused unwillingly and unknowingly by the mapper.
For these reasons it is vital that you compile as early as possible, and compile often.
Imagine you compile your map and during testing it turns out the lighting is completely broken, everything is pitch black.
If the previous compile was yesterday, you can easily retrace your steps: "What have I added today that could be causing this?".
Whereas if your last compile was 2 months ago, you will be scratching your head and problem-hunting for hours.
It happened to a contestant in 2019. It can happen to you. Play it safe! Compile often!
Gameplay is tricky to get right. The best strategy is to build a very simple version of your map, and have it tested as early as possible.
Your map is much easier to adapt in the early stages. Once everything is fully detailed, it becomes a real chore to change or move any gameplay elements around.
Therefore release early, rudimentary versions of your map and have them playtested! If you need help with playtests, contact staff.
GOOD strategy: Create simple map -> playtest -> refine w/ feedback -> playtest -> repeat until gameplay is fun -> detail aesthetically
BAD strategy: Create detailed map (work in secret for 3 months) -> Release and pray that the gameplay is fun
Small tip: When creating many versions, simply use the beta suffix; ie bg_yourmap_b1, bg_yourmap_b2 etc.
Each team having a "first flag" allows for climactic moments in gameplay; where one team must defend against a fullcap while the other team must push through to victory. Having a 2 person flag adds focus to the map and allows for decapping strategy and for fights to go back and fourth across the map. Single flag only maps sometimes have the shortcoming of being too easily fullcapped.
Choke points serve to connect your objective, they also allow for players to hold off larger amounts of players. Try not to have too many connection points to your objective.
The map size should be not be too large, running from one side of the map to the other should generally not take longer than a minute. Avoid having straight lines of sight that allow you to see extremely far away. This is bad for both performance and gameplay. Shorter lines of sight allow your map to "feel" larger without having tedious walking times.
Try to make use of verticality. Ramps, split levels and height variations allow for more interesting gameplay. Do not use ladders, as they are unwieldy and unusable in a competitive environment.
Avoid confined close quarters combat areas like, houses, rooms and narrow streets; these are not conducive to good linebattle gameplay.
Verticality in linebattle maps is extremely important, higher points in the map are strategically important. Try not to have your hills to be too small, a 15 person line should be able to fit comfortably on top of most of the terrain features on your map.
Try to have smooth transitions in height. Sharper transitions at times are harder to traverse, and could lead to issues of lines being in cover unintentionally.
Try to use foliage models that accurately reflect the season. Vary the orientation so the foliage looks organic. Do not clutter the map with props, these could be inadvertently used as cover for lines.