We give feedback from anything you provide us with, whether it's gameplay testing, screenshots or whatever. Remember to take feedback with a grain of salt, as people are always responding according to what they feel like is right, but as with every other art, there is no objective truth.
When designing maps, we usually do it in the following stages:
WIP/Planning: When you are designing to first rough version of the maps layout. In this phase, your map isn't ready for gameplay
Alpha: Map is ready for gameplay, but lacks aesthetics (to make it easier to change the layout). In this phase, your map should be tested a lot so you can improve the layout and game configs.
Beta: When your layout seems solid, you can move on to Beta, where you will detail your map.
Release candidate: When everything seems good, you move on to RC. Maps will usually stay in RC for a very long time. This is where you let people find the last tiny buggers, something that can take a while.
Final: When you are certain that your map requires no further work, you can call it final, whip the sweat of your forehead and be proud over your now complete map.
TF2maps offers gametesting for all maps on alpha/beta stage. Look out for the gameday announcements when you need to get your map tested

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mvm_sludge |
koth_nippletwister
Works in progress (inactive, or haven't reached a satisfying playable state yet): ctf_hurricane, cp_glacius, koth_sprout, pl_thorndike, tc_feldspar