階段

How it works

The object particle を (object) marks 階段(かいだん) (stairs) as what gets climbed. The verb 上がる(あがる) (go up) appears in its て (te-form) form 上がって(あがって) (going up), which here leaves the sentence hanging — it continues an action or invites a next step (“go up the stairs, and…”) rather than closing it off.