The Wheeler residence has a Basement beneath the house. The basement is the main hangout area of the Party. It is where they used to run their Dungeons & Dragons campaigns and have sleepovers. When the kids stumbled upon Eleven, Mike hid her in a pillow fort that he put under a table in his basement.
Besides the miscellaneous household stuffs, the Wheeler Basement contains a majority of Mike's toys along with his Supercom and the D&D campaigning equipment. The pillow fort is also situated here.
When Mike and his friends found and brought Eleven to the basement, Mike constructed a pillow fort so that she could sleep in for the night. Afterwards, when Eleven implied him of the dire situation she is in, Mike, despite Dustin & Lucas's cautioning, let her stay there. It is here where Eleven proved a mournful Mike that Will was still alive. Since then, the two fervently trusted each other and formed a strong bond.
Ever since Eleven vanished away after a showdown with the Monster to protect Mike and his friends, Mike used to go to the pillow fort and call for her every night with his Supercom, unaware that she was actually alive and was attempting to contact him too.