Hakka's most obvious archaic feature is the syllable-final stops, as in Yue/Cantonese and Min. I know Yue dissimilated some of them (details fuzzy in my memory, possibly -t went to -k with a dental initial). I don't know of any changes to them in Hakka.

Hakka retains the -m final, instead of merging it to -n. It retains distinct 'round' and 'sharp' (velar and dental) onsets in positions where some Mandarin varieties (including Standard) merge them to the new palatal j/q/x.

The Middle Chinese palatal ny- is lost in Hakka as it is almost everywhere. It merged with r- in Mandarin, y- in Yue, and ng- in Hakka. So you can't read the archaic form directly off any one of those varieties, but you can detect it by comparing any two.

Hakka tonal developments are middle-of-the-road. Like every variety of Chinese, it had the four Middle Chinese tones altered by the voicing feature of the syllable onset, and like every variety outside of Wu/Shanghainese, it then lost the voicing feature itself.

Hakka splits the ping/level tone in two by voicing, like almost every variety does. It splits the ru/entering 'tone' (the syllables with final stops) by voicing, like almost every non-Mandarin variety does, instead of disintegrating it like Mandarin.

In the shang/rising tone, Hakka agrees with Mandarin against Wu and Yue that you don't just split the tone down the middle by voicing; you split off voiced obstruent onsets and leave voiced sonorant onsets alone. It agrees with Mandarin and Wu against Yue that the split-off voiced onsets (whether all of them or just the obstruents) merge into the qu/departing tone.

Hakka agrees with Mandarin against almost everybody else that the qu/departing tone does not split.