Yep, that's the reactor! You're probably wondering how it got there...

[@Bookslayer10]

"Sure, my teammate is a bit new at the game, but I can just fight 1v2. What's the worst that could happen?"

[@Dark_Slayer3000]

At this point you gotta build mortars to defend against your teammates falling reactor xD

[@Bobleryobl]

Where can i get the buzzsaw?

[@EarthWorkGames]

The buzzsaw is available in the Moonshot DLC

[@Bobleryobl]

Thanks​@@EarthWorkGames