Is it actually warm enough for winter?

The walls are foam-insulated and the roof is asphalt, and a 30W heater is included. That combination holds heat far better than an unheated plastic shelter, but in hard freezes add straw bedding and face the door away from wind.

Does the heater need electricity?

Yes. The 30W heater runs on a cord, so you need an outdoor outlet or a weather-rated extension cord. Without power it still works as an insulated, unheated shelter.

The maker lists 1 to 3 cats. Two average adults fit comfortably; three is tight and better suited to smaller cats or kittens.

Can it be used indoors?

Yes, the knock-down panels suit indoor use too, though at about 30 lbs it is heavier than most indoor cat houses. The heater is mainly useful outdoors in cold weather.

How much assembly is required?

It ships as knock-down panels and assembles with basic tools. Expect a longer setup than a snap-together plastic house, and two people help when lifting the finished unit.