Building a playhouse - Part Two

Here's my basic plan: build a 10' x 10' base. Walls on all four sides. Front wall will have a door and maybe (depending on space) a window. Each side wall and the back wall will have a window. Build another 10' x 10' platform on top for the roof. Put 4x4s through that deck at all four corners and at edges of openings to support railing.

Issues still to resolve:

What kind of siding? Paint?
Window edging - simple flat 1x4s?
Floor - plain plywood? Something more gentle on feet?
Snow on roof - a potential issue up here in the frigid north.
Make the roof solid or let it drain through? (If it drains, the floor needs to drain too.)
If solid roof, should I put a slight angle on it for drainage?

Building a playhouse - Part One

We have two little ones: W, a boy, is coming up on three and C, a girl, is coming up on one. I want to build a playhouse! I've browsed the plans (both free and for purchase) on the web and am unimpressed. Here are the attributes I'm looking for:

It's not going to be a treehouse, so there needs to be some element of climbability. So why not make it a flat roof? With a ladder up from the inside through a trap door! Terrific. Need to make sure the trap door won't slam shut on small fingers. Need a railing around the roof/deck. Let's add a slide off the roof - create an opening in the railing. I'm also thinking a rope net would be fun for climbing up. How about some rock climbing grips up one wall? This is going to be fun!