It's fine to start without defense plans. The first pressure is to get some energy into your controllers within about 24 hours, which is fairly simple.

When you can, build a tower, give it energy, and write code to have it attack things.  Towers are extremely strong defense.

In terms of starting locations, the farther away from other players the better.  If you change the map to show you "energy mined in last hour", pick a spot where nobody is mining, and ideally where nobody is mining even adjacent to that.  It'll give you some good buffer.

Alternatively, look for rooms with a highlighted border - those are newbie spawns, and being next to them is reasonably safe in comparison to someone with high level murder bots.

Awesome, it was basically impossible to take a room once a tower went up.

How long does a change usually stay on PTR be moving to live?

I'm in https://screeps.com/a/#!/room/E19S18, and would love to purge the evil threat at https://screeps.com/a/#!/room/E18S19, but the tower is amazingly strong.

It appears to do 400HP of damage at full range every tick for 10 energy, which burns through my 3000 HP murder bots before they can get in range of any real target.  

Sending in a bunch of tough (tough: 8, move: 2) or cheap (move: 1) bots couldn't beat the refill rate on the tower.

Is it feasible to take out a towered room without power creep?