Learn Spanish (Latin America): Ellos andan moldeando y compactando las murallas. - They go around molding and compacting the ramparts.

Using andar followed by a gerund (like moldeando) creates a diffuse or intermittent progressive action. While están moldeando means they are doing it right this exact second in one spot, andan moldeando implies a continuous but scattered action. It gives the feeling that they are going around doing this task over a period of time, stopping and starting, or moving along different parts of the ramparts.

Do I need to repeat andan before compactando?

No, you do not. When you have two or more gerunds (verb forms ending in -ando or -iendo) connected by y, you only need to use the auxiliary verb andan before the first one. It automatically applies to both actions in the sentence.

Why use murallas here instead of paredes or muros for walls?

Muralla specifically refers to a large, thick, defensive wall, like a rampart around a castle, fortress, or ancient city. A pared is an everyday wall inside or outside a building, and a muro is usually a freestanding outdoor wall or barrier. Since they are working on massive defensive ramparts, murallas is the precise word.

The verb is moldear. Why is the gerund moldeando rather than moldeiendo?