The core is an isoquinoline ring: a six-membered pyridine ring (six-membered aromatic ring bearing one ring nitrogen) fused to a benzene ring.

Choose a benzene carbon ortho to the fusion junctions as position 1; it bears a chlorine atom.

Number around the benzene ring 1 -> 2 -> 3 -> 4, then (skipping the fusion carbons) continue through the fused pyridine portion 5 -> 6 (nitrogen) -> 7 -> 8.

At position 5, the ring carbon carries an exocyclic -NH-C(=NH)-NH2 group (i.e., the ring carbon is bonded to NH, which is attached to the central carbon of a guanidine moiety).

No stereocentres or E/Z double bonds are present.