You can read through the schema documentation and get an idea of what fields are required to make the database. You'll need the Indoors license to use their data management tools and pre-configured apps, but you can make an Indoors compliant database without using those licensed tools.
The minimum fields needed for the room layer (called Units in Indoors) are LEVEL_ID (for the floor filter), UNIT_ID (unique room number), and FACILITY_ID (building the floor is in). Other layers that are required to make things works include FACILITIES (building footprint), SITE (overall collection of facilities), and LEVELS (individual floor outlines). All of those require a portion of those minimum fields listed above (e.g. LEVELS would have all but the UNIT_ID field).
Justin Anderson wrote a blog post about how to use floor plans without an Indoors license, so that might be worth looking up and reading through.
