Re: Did you watch the same episode I did?
The axe and the tank were a punchline. The setup was Missy's line about finding the Doctor by looking for "even the slightest anachronism..." which trails off as the first power chords split the air. Classic subversion of expectations. It was actually a double punchline - first The Doctor emerges with an absurd anachronism - the electric guitar - then an even dafter one emerges - the tank. Brilliant.
And the handmines were great as well. Straight out of a small boy's nightmares, which is entirely the point.