For a simple game, this looks great! One thing I could recommend is to move the map away from the lobby and maybe include a spectate mode for players who died.
Maybe also modernize the UIs and play with new icons, sounds, etc.
I would like to give my overall feedback! The game went really smooth, and the round system worked very well.
Here are the things that you could improve on:
GUI: The most important thing is… they worked! Every button that was meant to be pressed worked. Maybe you could implement some tweening to the GUI to make it look more professional. The things you could improve on is the quality of the UI images and maybe give the GUI rounded edges, something you can easily do with the plugin Roundify.
Architecture and Terrain: The maps were nicely built, though the game lacked in variety of different maps. Like what Xarbleed said, maybe move the lobby a little bit farther from the pvp maps, and include a spectate mode.
Overall, GUI worked but needs a little polishing up by modernizing (click sound effects, smooth edges, tween animations) and a bigger variety of maps.
I would make the spawn fully transparent so it can’t be seen. You should also try and make the GUI look a little bit better and clenaer. Lastly I would add details to to the map to make it look better such as another small building.