I find the character of General Cutler incredibly interesting. The story essentially becomes about him in episode 3 - he is credited second after Hartnell - because that episode shows his transformation into a Cyberman! 

The Cybermen are fighting for survival, don't care about masses of people dying and are prepared to destroy another planet to save their own. Whereas Cutler is fighting for survival, doesn't care about the possibility of masses of people dying (he thinks it's a risk worth taking when using the Z-bomb) and is prepared to destroy Mondas to save the Earth. Just like Earth and Mondas parallel each other - Cutler and the Cybermen parallel each other. The way Cutler takes out Ben at the end of episode 3 is even similar to how the Cybermen attack people in the story! The only difference is that Cybermen are devoid of all emotion, whereas what fuels his Cutler's desire to use the Z-bomb is the fact his son is sent into space on a probably fatal mission. 

'Come on man, admit it. I am. I'm scared for that son of mine.' 

He even admits that he's scared. Fear is key to this story, since it is also what drives Barclay to help Ben and Polly sabotage the bomb. However, Cutler's fear makes him react aggressively. Also, since we know at this point that Mondas is likely to burn up if they wait, everyone is against him and Cutler essentially becomes the villain for a while - we are sort of relieved when he is killed at the start of episode 4. This is pretty similar to Midnight in series 4 of New Who actually, with the passengers reacting in the most drastic way possible even though help is on its way. However, the Cybermen are a lot more hostile than that creature was... Anyway, this story goes to a great effort to highlight 'ends justify the means' leaders are not in the right.

For the final part of the story Cutler dies which allows the Doctor to take command - which is brilliant for his final ever episode and Hartnell gets a real time to shine. He is then taken to their ship to join Polly as hostages, but the Cybermen are stalled long enough to allow for Mondas to melt. 

I also wouldn't say the Cybermen feel scary at all though, instead they feel cruel. If anything it's the ruthlessness of Cutler that is actually scarier. The Cybermen do feel incredibly easily killed though, especially with Ben blinding one of them with a projector. Also, what is it with 60s iterations of Doctor Who monsters being reliant on some sort of energy? First we have the Daleks reliant on electricity, and here we have the Cybermen dependent on energy from their planet Mondas. The science behind this story is a bit dodgy as well. Why did the Cybermen want to destroy the Earth? Why didn't they just stop Mondas absorbing the energy once they have got enough? Especially since they explain to the Cybermen that the destruction of Mondas is going to happen, surely they'd just leave and move Mondas out the way? How were they draining energy from Earth in the first place? 

Like I said though, I was incredibly surprised by how much I got out of this. Despite there being some weird stuff in the earlier half of the story. Like that man with skimpy female posters on the wall being weird about Polly and that between the legs shot(?) of one of the Cybermen in episode 2. I also think as soon as find we find out that Mondas will be destroyed from absorbing too much energy the tension is broken, since we know how it’s all going to be resolved, and there’s not even a sense of the fighting them off until daylight trope like zombies since there’s not many of them and they’re so easily defeated.

I also like how the Doctor is very knowing here - working out the problem of there being a twin planet and the solution of its destruction long before everyone else. He seems very much like he is one step ahead of everyone and it’s just as well that he passes out in episode 3, otherwise he probably would have solved the story too quickly.

But why did the Doctor regenerate? Who knows! One can only assume in draining energy from the Earth the Cybermen also drained energy from the Doctor.

8/10