Does the trundle take a standard twin mattress?

Yes. The trundle is sized for a twin mattress, the same size as the two bunks. A thinner mattress, around 6 inches, keeps it rolling in and out easily.

How many people can it sleep?

Three. Two sleep on the stacked twin bunks and one on the pull-out trundle. The trundle is best for a child or a shorter adult.

Is the frame solid wood?

The frame is pine, a solid wood. Pine is softer than oak or maple, so it can show dents and scratches with rough use.

Do the storage drawers fit under the lower bunk?

Yes. The drawers are built into the base under the lower bunk. Leave clearance in front so they can slide out fully.

Is a mattress included?

No. Mattresses for the two bunks and the trundle are sold separately. Use twin mattresses, and keep the upper bunk mattress within the height marked on the guardrail.

How hard is assembly?

It is a substantial build with many parts and hardware. Two adults should plan a few hours, and a second person helps when lifting the upper bunk into place.