hole, doorway, ditch
竇 centers on an opening or passage: a hole, a doorway, or a ditch. These meanings share the idea of a gap or channel that allows passage or drainage.
竇 combines 穴 (hole, cave) with 賣 (sell), which likely acts as a phonetic component. The exact historical development is uncertain, but the structure suggests a phono-semantic compound where 穴 provides the meaning of an opening and 賣 contributes the sound.
穴 (hole) on top of 賣 (sell) – imagine a merchant selling goods through a hole in the wall, which serves as a doorway or a ditch-like passage for transactions.
For トウ, picture a toll booth at a doorway: you pay a toll (トウ) to pass through the hole. The toll sound cues トウ while the hole represents the kanji's meaning.