The figure on Christ's left is St John the Evangelist, the only one of the twelve disciples who was present at the Crucifixion, and to whom the dying Christ bestowed His Mother. St John is also always present with the Mother of God in conventional icons of the Crucifixion.
The script within the horizontal arm of the cross. under Christ's arms, is one of the troparia to the Cross: We bow down before Your Cross, O Master, and we glorify Your holy Resurrection. The Slavonic version is Кресту Твоему поклоняемся, Владыко, и святое воскресение Твое славим.
The angels are a common feature of Crucifixion icons, expressing their grief and incomprehension at the death of their God and Master, yet glorifying His Passion. These verses from the Lamentations of Great Friday expresses it well:
In the tomb they laid You, O Christ, who is Life; in amazement, angelic armies lift up their song as they glorify Your self-abasement, O Lord.
Angels are bewildered, and the bodiless hosts at a loss, O Christ, before that great mystery: Your ineffable entombment, beyond speech.
To the angels, O Saviour, You are gladness and joy, but a cause of grief You now have become to them, as they see You in the flesh, a lifeless corpse.
The text on the face of the cross between the angels reads Angel(s) of the Lord; that just above the short horizontal bar of the cross reads The King of Glory; and that on either side of the inscription IC-XC reads The Son of God.
The building behind Christ's feet represents the wall encompassing the city of Jerusalem. This, too, is often seen in Crucifixion icons. It illustrates not only the fact that Christ was crucified outside the city, but, if memory serves, points to OT prophecies of the same.
On the back of the cross, starting from the top:
Let us love one another
The troparion to the Cross, as above
On the long horizontal arm, a prayer to the Cross, incorporating Paschal verses:
Let God arise, and let His enemies be scattered; and let those who hate Him flee from His face. As smoke vanishes, let them vanish; and as wax
melts from the presence of fire, so let the demons perish from the presence of those who love God and who sign themselves with the Sign of the Cross and say with gladness: Hail, most precious and life-giving Cross of the Lord, for you drive away the demons by the power of our Lord Jesus Christ who was crucified on you, went down to hell and trampled on the power of the devil, and gave you to us, His honourable Cross, for driving away all enemies. O most precious and life-giving Cross of the Lord, help me with our holy Lady, the Virgin Mother of God, and with all the Saints throughout the ages. Amen.
On the vertical arm above the slanted bar, the Exaposteilarion from the service of the Elevation of the Cross (September 14):
The Cross is the guardian of the whole world; the Cross is the beauty of the Church; the Cross is the might of kings; the Cross is the confirmation of the faithful; the Cross is the glory of angels and the plague of demons.