You don't really 'need' to open the door, it's enough to destroy all exits. Question is how cheap you can make biped drone and how cheap you can make a small rover capableof moving 10-15 kilos of explosives. Rover can be made as cheap as <$500 or so.
Artillery includes rocket artillery, but even 'regular' gun artillery can shoot for tens of kilometers with specialised shells, while being mobile at that.
anecdote
I've been of the opinion that arty is basically guns but big nothing special about it, but whitnessing modern artillery piece work changed it considerably, while yes they are just big guns, but they are big guns. Like you feel it in your bones when it shoots.