Letters with circumflex are long in Welsh -- ones without can be short or long (partly depending on the word shape).
For example, twr "pile; tor" has a short vowel, tŵr "tower" has a long vowel.
llyn "lake" has a short vowel, llun picture has a long vowel -- there, you can't tell just from the spelling that llun will rhyme with (Pen) Llŷn rather than with llyn.
Sometimes, short vowels are marked by a grave accent, e.g. bòs for "boss".