あれほど sense 3
あれ程 explicitly means 'to that extent', while あそこ can colloquially carry the same degree/extent meaning in casual speech.
あれN5
あそこ specifically means 'over there' as a location, while あれ is vaguer and can stand for a thing, person, time, or place.

Opposites

ここN5sense 1
ここ refers to a place near the speaker, while あそこ refers to a place far from both speaker and listener.
そこN5
そこ refers to a place near the listener, while あそこ refers to a place far from both speaker and listener.

Don't confuse with

底そこN3sense 1
そこ and あそこ are both demonstratives for 'there' and are easy to mix up, but そこ points to a place near the listener while あそこ points to a place far from both speaker and listener.
どこN5
あそこ means 'over there,' a specific known place far from both speaker and listener, unlike どこ which asks which place.

Written forms

あそこUsage85%
Standard kana spelling; the word is usually written in kana.
アソコUsage7%
kana-only spelling
Katakana spelling, sometimes used for emphasis or in informal writing.
かしこUsage5%
Archaic or formal variant meaning 'there'; rarely used in modern speech.
あすこUsage3%
Dialectal or older variant of あそこ; not common in modern standard Japanese.

Etymology

Derived from the demonstrative prefix あ (distant from both speaker and listener) + そこ (place). The exact historical development is uncertain, but it is part of the ko-so-a-do demonstrative system.

彼処Usage1%Usage for this spelling across all readings, not one reading alone.

rarely-used kanji form

あそこ
Rarely used kanji form; learners should recognize it but use kana.
かしこ
あしこ
out-dated reading
あすこ

彼所Usage<1%Usage for this spelling across all readings, not one reading alone.

rarely-used kanji form

あそこ
Rarely used kanji form; learners should recognize it but use kana.
かしこ
あしこ
out-dated reading
あすこ

あしこ

out-dated reading

Obsolete kana usage; not used in contemporary Japanese.