See on Act 17:23. The altar of sacrifice, as is indicated by slain; not the altar of incense. The imagery is from the tabernacle. Exo 39:39; Exo 40:29.
Souls (ψυχὰς)
Or lives. See on 3Jo 1:2. He saw only blood, but blood and life were equivalent terms to the Hebrew.
Slain (ἐσφαγμένων)
See on Rev 5:6. The law commanded that the blood of sacrificed animals should be poured out at the bottom of the altar of burnt-offering (Lev 4:7).
They held (εἶχον)
Not held fast, but bore the testimony which was committed to them.