Wow, you people are still arguing over this? Me and the Original Poster settled this like... on page two?

It's a game mechanic. An intended game mechanic. Peacetime is not usable whilst an attack is coming.
Teleporting comes at a disadvantage, you can no longer protect your valley's or other cities.

The fact of a troop attacking a coord comes as basically the villian in the movies;

Joker places a bomb just below the cat-walk Batman is -sure- to cross, with his precious side-kick Robin tied up on the other side. Batman quickly heads for his friend.
Batman; I'm on my way!
Joker; Ahhh, you won't catch me! *Flee*
Batman; I must save Robin! *Bolts over the catwalk and goes boom*
Batman; Arg! I'm wounded, but I'm still on my way!

In a sense, you lost your troops. Woo. Build more?
Alternate universe:

Batman; I must save Robin! *Bolts over the catwalk and hears a ticking, quickly ducking back as it goes boom. He jumps over and gets Robin anyways, unhurt*

Hooray! You realized they moved! You kept yourself from being blown up.



....The end.

Edit; And on the Valley's changing... Uhh, they gradually die/build up over time. If they didn't, you'd have to actually -Look- for a certain level valley if all the ones nearby are level 10, permanently.