Romans From Mars is a simplistic shooting game that plays kind of like a tower defense, but not really. You fire a ballista at incoming waves of enemies before they reach your wall. Enemies that aren't killed in time deal one damage to your wall, and you lose if the wall reaches zero health.
This game is a textbook example of the cash-grab mobile game ported to PC. It quickly spikes in difficulty (level 19 is a good example) and sets up situations where you don't have enough currency to keep up unless you grind slowly or pay money. More annoying than that, the game doesn't always register mouse clicks (or perhaps somehow delays them), giving enemies some free time to advance at the start of a level and typically ruining that particular attempt altogether.
It's also fairly limited on defense options, with upgrades to the ballista, wall health, and a magic spell to freeze (or burn) enemies. While the title plays somewhat like a tower defense game, it's missing staples such as autofire turrets, mines/traps and barricades, to name a few.
You should probably skip Romans From Mars entirely. There's better shooters, tower defense games and mobile ports available, even for free.